Chemically strengthened glass and method for manufacturing chemically strengthened glass

ABSTRACT

A chemically strengthened glass having a thickness t of 2 mm or less, having a compressive stress value (CS 90 ) at a portion of 90 μm deep from a glass surface being 25 MPa or more, and satisfying a number of fragments generated within a size of 25 mm×25 mm being 20 or less in a fracture test according to an indenter indentation test under a condition of holding a load ranging from 5 kgf to 10 kgf for 15 seconds with a pyramidal diamond indenter having an indenter angle of a facing angle of 60°.

TECHNICAL FIELD

The present invention relates to a chemically strengthened glass and amethod for manufacturing a chemically strengthened glass.

BACKGROUND ART

In recent years, in order to enhance protection and good appearance ofdisplay devices of mobile devices such as mobile phones, smart phones,personal digital assistant (PDA), and tablet terminals, a cover glasscomposed of a chemically strengthened glass has been used.

In particular, in the mobile devices such as smart phones, generation ofa deep flaw when they are dropped by mistake and collide with acollision object having a small-angle colliding part (hereinafter alsoreferred to as an acute-angle object) such as sand, and development of aflaw due to a generation of tensile stress on the glass surface in theflawed state are prone to occur. Thus, there are relatively manyopportunities where breakage of the chemically strengthened glass as acover glass occurs. Accordingly, a chemically strengthened glass whichis hardly broken even in the case of collision with an acute-angleobject (hereinafter also referred to as “having a high acute-angle flawstrength”) is desired.

It is considered that, as the glass having a high acute-angle flawstrength, a chemically strengthened glass having a deep compressivestress layer (DOL) is advantageous, and in order to enhance productionefficiency, a glass showing a large ion exchange rate has been desired.

Patent Document 1 discloses that a chemically strengthened glass havinga large depth of the compressive stress layer (DOL) has a high fractureresistance in a drop test using #180 sandpaper.

Patent Document 2 discloses a lithium aluminosilicate glass showing afast ion exchange rate.

Furthermore, Patent Document 1 discloses a chemically strengthened glassthat uses both of Na—K exchange where Na ions in the glass are exchangedwith K ions and Li—Na exchange where Li ions in the glass are exchangedwith Na ions.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: US-A-2015/0259244-   Patent Document 2: JP-T-2013-520385    (the term “JP-T” as used herein means a published Japanese    translation of a PCT patent application)

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, the present inventors have extensively studied, and it wasfound that, when a mobile device such as a smart phone is dropped bymistake at a practical scene such as on the street, the depth of theflaw generated on the cover glass by the collision with an acute-angleobject such as sand is deeper than the flaw generated in a drop testusing #180 sandpaper. Therefore, in the cover glass composed of achemically strengthened glass, at a practical scene, the resistanceagainst fracture caused by the flaw generated in the drop test using#180 sandpaper as described in Patent Document 1 is not necessarilysufficient. Moreover, in the chemically strengthened glass that uses theNa—K exchange as described in Patent Document 1, it requires a long ionexchange treatment time such as 10 hours or more to obtain a large depthof the compressive stress layer, such as more than 100 μm.

Though Patent Document 2 discloses a lithium aluminosilicate glassshowing a fast ion exchange rate, a guideline for enhancing strengthagainst the acute-angle object flaw is unclear.

As above, specific methods for enhancing the acute-angle object flawstrength has hitherto not been clear. For satisfying a high acute-angleobject flaw strength, a glass having a deeper depth of the compressivestress layer and a larger compressive stress value of the compressivestress layer is necessary. Moreover, it is simultaneously required thatin the case of breakage, the number of the glass fragmentation is smalland a safety is high. In order to obtain such a glass, a glass showing alarge ion exchange rate necessary for chemical strengthening is desired.

Accordingly, an object of the present invention is to provide achemically strengthened glass which is excellent in the acute-angleobject flaw strength and also has a high safety at fragmentation andshows a fast ion exchange rate.

It is known that the cover glass is broken by a cause of surface tensilestress generated at the time of drop or by the collision with an object,and also is broken due to a flaw resulting from the collision of thesurface with an acute-angle object, and the like. In order to improvestrength, it is necessary to enhance both of bending strength andbending strength after flawing with an acute-angle object (hereinafteralso referred to as “bending strength after flawing”). Here, in order toenhance both of the bending strength and the bending strength afterflawing, the following four items are required: (1) introduction oflarger compressive stress as far as possible to the outermost surface ofthe glass, (2) introduction of deeper compressive stress layer as far aspossible to the glass, (3) introduction of larger compressive stress asfar as possible to the inside of the glass as deep as possible, and, inorder to achieve (2) and (3), (4) a large ion exchange rate.

Here, in Patent Document 1, though the above (1), (2), and (4) areachieved by using both of the Na—K exchange and the Li—Na exchange, itis insufficient in view of the above (3).

In consideration of the above conventional problems, an object of thepresent invention is to provide a chemically strengthened glass havingboth of a high bending strength and a high bending strength afterflawing and a method for manufacturing the chemically strengthenedglass.

Means for Solving the Problems

One embodiment of the present invention relates to a chemicallystrengthened glass having a thickness t of 2 mm or less,

having a compressive stress value (CS₉₀) at a portion of 90 μm deep froma glass surface being 25 MPa or more, and

satisfying a number of fragments generated within a size of 25 mm×25 mmbeing 20 or less in a fracture test according to an indenter indentationtest under a condition of holding a load ranging from 5 kgf to 10 kgffor 15 seconds with a pyramidal diamond indenter having an indenterangle of a facing angle of 60°,

in which the chemically strengthened glass provides a depth of acompressive stress layer (DOL) being 50 μm or more when a glass sheethaving a thickness of 1 mm which has a matrix composition of thechemically strengthened glass and has been gradually cooled from atemperature T° C., which is 30° C. to 50° C. higher than a glasstransition temperature, to (T−300)° C. at 0.5° C./minute, is subjectedto an ion exchange treatment for one hour with a molten salt at 400° C.composed of KNO₃, NaNO₃, or a mixed salt of KNO₃ and NaNO₃.

Moreover, it also relates to a chemically strengthened glass having athickness t of 2 mm or less,

having a compressive stress value (CS₉₀) at a portion of 90 μm deep froma glass surface being 25 MPa or more, and

satisfying a number of fragments generated within a size of 25 mm×25 mmbeing 20 or less in a fracture test according to an indenter indentationtest under a condition of holding a load ranging from 5 kgf to 10 kgffor 15 seconds with a pyramidal diamond indenter having an indenterangle of a facing angle of 60°,

in which the chemically strengthened glass provides a depth of acompressive stress layer (DOL) being 70 μm or more when a glass sheethaving a thickness of 1 mm which has a matrix composition of thechemically strengthened glass and has been gradually cooled from atemperature T° C., which is 30° C. to 50° C. higher than a glasstransition temperature, to (T−300°) C at 0.5° C./minute, is subjected toan ion exchange treatment for one hour with a molten salt at 425° C.composed of KNO₃, NaNO₃, or a mixed salt of KNO₃ and NaNO₃.

In the above chemically strengthened glass, any of bending strengthafter flaw σa, σb, and σc is preferably 150 MPa or more. Each of thebending strength after flaw σa, σb, and σc is a fracture stress value(bending strength, unit: MPa) obtained by performing four-point bendingtest under conditions of an under spun of 30 mm, an upper spun of 10 mm,and a crosshead speed of 0.5 mm/minute after the glass surface is flawedafter a diamond indenter (indenter angle of facing angle: 110°) ispressed thereto for 15 seconds at a load of 0.5 kgf, 1 kgf, and 2 kgf,respectively. The degree of tensile stress generated on the cover glasssurface at the time of drop of a smart phone is about 150 MPa. Then, inthe case where any of σa, σb, and σc is 150 MPa or more, fractureresulting from the stress generated by the drop even after theoccurrence of the flaw with an acute-angle object can be prevented.

In the above chemically strengthened glass, ΔCS₁₀₀₋₉₀ (unit: MPa/μm) ispreferably 0.4 or more, in which ΔCS₁₀₀₋₉₀ is calculated according tothe following expression using CS₉₀ and a compressive stress value(CS₁₀₀) at a portion of 100 μm deep from the glass surface.

ΔCS ₁₀₀₋₉₀=(CS ₉₀ −CS ₁₀₀)/(100−90)

ΔCS₁₀₀₋₉₀ (unit: MPa/μm) is preferably 4.0 or less.

In the above chemically strengthened glass, ΔCS_(DOL-20) (unit: MPa/μm)is preferably 0.4 or more, in which ΔCS_(DOL-20) is calculated accordingto the following expression using a compressive stress value CS_(DOL-20)at a depth closer to a glass surface side by 20 μm from DOL.

ΔCS _(DOL-20) =CS _(DOL-20)/20

ΔCS_(DOL-20) is preferably 4.0 or less.

In the above chemically strengthened glass, a surface compressive stressvalue (CS) is preferably 300 MPa or more.

In the above chemically strengthened glass, the depth of a compressivestress layer (DOL) is preferably 100 μm or more.

In the above chemically strengthened glass, the thickness t ispreferably 0.9 mm or less.

In the above chemically strengthened glass, an area Sc (MPa·μm) of acompressive stress layer is preferably 20,000 MPa·μm or more.

In the above chemically strengthened glass, a value Sc/t (MPa) obtainedby dividing an area Sc (MPa·μm) of the compressive stress layer by thethickness t (m) is preferably 28 MPa or more.

In the above chemically strengthened glass, the matrix composition ofthe chemically strengthened glass preferably contains, in terms of molarpercentage on the basis of oxides, from 50 to 80% of SiO₂, from 1 to 30%of Al₂O₃, from 0 to 6% of B₂O₃, from 0 to 6% of P₂O₅, from 0 to 20% ofLi₂O, from 0 to 20% of Na₂O, from 0 to 10% of K₂O, from 0 to 20% of MgO,from 0 to 20% of CaO, from 0 to 20% of SrO, from 0 to 15% of BaO, from 0to 10% of ZnO, from 0 to 5% of TiO₂, and from 0 to 8% of ZrO₂.

In the matrix composition of the above chemically strengthened glass,the content of ZrO₂ in terms of molar percentage on the basis of oxidesis preferably 1.2% or less.

The content of Na₂O in terms of molar percentage on the basis of oxidesis preferably 3% or more.

The content of K₂O in terms of molar percentage on the basis of oxidesis preferably 0.5% or more.

The content of B₂O₃ in terms of molar percentage on the basis of oxidesis preferably 1% or less.

The content of Al₂O₃ in terms of molar percentage on the basis of oxidesis preferably 11% or less.

Moreover, another embodiment of the present invention relates to achemically strengthened glass which satisfies the following expressions(1) and (2),

NM/Nh≥1.8  (1),

KM/Kh≥3  (2),

in which NM, Nh, KM, and Kh each represents the following:

NM: a value obtained by converting a maximum value in an Na ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to Na₂O (% by weight);

Nh: a value obtained by converting a central value of a sheet thicknessin the Na ion concentration profile in the sheet thickness direction ofthe chemically strengthened glass measured by EPMA to Na₂O (% byweight);

KM: a value obtained by converting a maximum value in a K ionconcentration profile in the sheet thickness direction of the chemicallystrengthened glass measured by EPMA to K₂O (% by weight); and

Kh: a value obtained by converting a central value of a sheet thicknessin the K ion concentration profile in the sheet thickness direction ofthe chemically strengthened glass measured by EPMA to K₂O (% by weight).

The above chemically strengthened glass preferably further satisfies thefollowing expression (3),

N0/Nh≥0.8  (3),

in which N0 represents the following:

N0: a value obtained by converting a value on an outermost surface inthe Na ion concentration profile in the sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight).

The above chemically strengthened glass preferably further satisfies thefollowing expression (4),

N0/NM≥0.4  (4),

in which N0 represents the following:

N0: a value obtained by converting a value on an outermost surface inthe Na ion concentration profile in the sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight).

Furthermore, still another one embodiment of the present inventionrelates to a chemically strengthened glass which satisfies the followingexpressions (5) and (6),

NM−Nh≥2.2 (wt %)  (5),

KM−Kh≥3 (wt %)  (6),

in which NM, Nh, KM, and Kh each represents the following:

NM: a value obtained by converting a maximum value in an Na ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to Na₂O (% by weight);

Nh: a value obtained by converting a central value of a sheet thicknessin the Na ion concentration profile in the sheet thickness direction ofthe chemically strengthened glass measured by EPMA to Na₂O (% byweight);

KM: a value obtained by converting a maximum value in a K ionconcentration profile in the sheet thickness direction of the chemicallystrengthened glass measured by EPMA to K₂O (% by weight); and

Kh: a value obtained by converting a central value of a sheet thicknessin the K ion concentration profile in the sheet thickness direction ofthe chemically strengthened glass measured by EPMA to K₂O (% by weight).

The above chemically strengthened glass preferably further satisfies thefollowing expression (7),

N0−Nh≥−0.4 (wt %)  (7),

in which N0 represents the following:

N0: a value obtained by converting a value on an outermost surface inthe Na ion concentration profile in the sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight).

The above chemically strengthened glass preferably further satisfies thefollowing expression (8),

N0−NM≥−3.5 (wt %)  (8),

in which N0 represents the following:

N0: a value obtained by converting a value on an outermost surface inthe Na ion concentration profile in the sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight).

In the above chemically strengthened glass, a compressive stress value(CS₂₀) at a portion of 20 μm deep from the glass surface is preferably60 MPa or more.

In the above chemically strengthened glass, a compressive stress value(CS₄₀) at a portion of 40 μm deep from the glass surface is preferably60 MPa or more.

In the above chemically strengthened glass, a bending strength ispreferably 400 MPa or more.

In the above chemically strengthened glass, a surface compressive stressvalue (CS) is preferably 400 MPa or more.

In the above chemically strengthened glass, a depth of a compressivestress layer (DOL) is preferably 100 μm or more.

The above chemically strengthened glass is preferably a sheet-form witha thickness t of 2 mm or less.

In the above chemically strengthened glass, a number of fragmentsgenerated within a size of 25 mm×25 mm is preferably 20 or less in afracture test according to an indenter indentation test under acondition of holding a load ranging from 5 kgf to 10 kgf for 15 secondswith a pyramidal diamond indenter in which a facing angle is 60°.

In the above chemically strengthened glass, a matrix composition of thechemically strengthened glass preferably contains, in terms of molarpercentage on the basis of oxides, from 50 to 80% of SiO₂, from 1 to 30%of Al₂O₃, from 0 to 5% of B₂O₃, from 0 to 4% of P₂O₅, from 3 to 20% ofLi₂O, from 0 to 8% of Na₂O, from 0 to 10% of K₂O, from 3 to 20% of MgO,from 0 to 20% of CaO, from 0 to 20% of SrO, from 0 to 15% of BaO, from 0to 10% of ZnO, from 0 to 1% of TiO₂, and from 0 to 8% of ZrO₂.

In the matrix composition of the above chemically strengthened glass,the content of ZrO₂ in terms of molar percentage on the basis of oxidesis preferably 1.2% or less.

The content of Na₂O in terms of molar percentage on the basis of oxidesis preferably 3% or more.

The content of K₂O in terms of molar percentage on the basis of oxidesis preferably 0.5% or more.

The content of B₂O₃ in terms of molar percentage on the basis of oxidesis preferably 1% or less.

The content of Al₂O₃ in terms of molar percentage on the basis of oxidesis preferably 11% or less.

Moreover, the present invention also relates to a method formanufacturing the above chemically strengthened glass includingperforming at least two stages of ion exchange treatments on a glass, inwhich KNO₃ concentration in a molten salt to be used in a first-stageion exchange treatment is 60% by weight or more and NaNO₃ concentrationin a molten salt to be used in a second-stage ion exchange treatment is5% by weight or more.

Advantageous Effect of the Invention

An embodiment of the chemically strengthened glass of the presentinvention is excellent in acute-angle object flaw strength and also hasa high safety at fragmentation and shows a short ion-exchange time.

An embodiment of the chemically strengthened glass of the presentinvention also has a high bending strength and a high bending strengthafter flawing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an outline view illustrating a manner for manufacturing asample for measuring surface compressive stress (CS) of a chemicallystrengthened glass, and (a) shows a sample before polishing and (b)shows a thinned sample after polishing.

FIG. 2 is a conceptual view illustrating a stress profile of achemically strengthened glass for explaining CS (surface compressivestress value), DOL (depth of surface compressive stress layer), CT(internal tensile stress), and St (area of internal tensile stresslayer).

FIG. 3 shows a schematic view illustrating a test method for an on-sanddrop test.

FIG. 4 is a graph plotting a relationship between a compressive stressvalue CS₉₀ (unit: MPa) at a portion of 90 μm deep from the grass surfaceand an average crack height (unit: mm) for the chemically strengthenedglass of each example.

FIG. 5 is a graph plotting a relationship between ΔCS₁₀₀₋₉₀ (unit:MPa/μm) and fracture stress (MPa) under each flawing condition for thechemically strengthened glass of each of Examples 4, 6, 8 to 10, and 15.

FIG. 6 shows a graph plotting a relationship between bending strengthafter flawing under a condition of a load of 0.5 kgf or 1 kgf and CS₂₀.

FIG. 7 shows a plot of a relationship between bending strength afterflawing under a condition of a load of 0.5 kgf or 1 kgf and CS₄₀.

FIG. 8 shows a graph plotting a relationship between NM/Nh and CS₂₀ orCS₄₀.

FIG. 9 shows a graph plotting a relationship between N0/Nh and CS₂₀ orCS₄₀.

FIG. 10 shows a graph plotting a relationship between N0/NM and CS₂₀ orCS₄₀.

FIG. 11 shows a graph plotting a relationship between NM−Nh (unit: wt %)and CS₂₀ or CS₄₀.

FIG. 12 shows a graph plotting a relationship between N0−Nh and CS₂₀ orCS₄₀.

FIG. 13 shows a graph plotting a relationship between N0−NM and CS₂₀ orCS₄₀.

MODE FOR CARRYING OUT THE INVENTION

The following will describe the chemically strengthened glass of thepresent invention in detail.

The chemically strengthened glass of the present invention is achemically strengthened glass I to be described in the following or achemically strengthened glass II to be mentioned below.

In the chemically strengthened glass I of the present invention, acompressive stress value (CS₉₀) at a portion of 90 μm deep from theglass surface is 25 MPa or more and the number of fragments generatedwithin a size of 25 mm×25 mm is 20 or less in a fracture test accordingto an indenter indentation test under the condition of holding a loadranging from 5 kgf to 10 kgf for 15 seconds with a pyramidal diamondindenter in which an indenter angle of the facing angle is 60°.

The chemically strengthened glass I of the present invention has acompressive stress layer formed by a chemical strengthening treatment(ion exchange treatment), on the surface. In the chemical strengtheningtreatment, the surface of a glass was ion-exchanged to form a surfacelayer where compressive stress remains. Specifically, by ion exchange ata temperature equal to or lower than the glass transition temperature,alkali metal ions being present in the vicinity of surface of the glasssheet and having a small ionic radius (typically Li ions or Na ions) aresubstituted with alkali ions having a larger ionic radius (typically Naions or K ions for Li ions and K ions for Na ions). Thereby, compressivestress remains at the glass surface and the strength of the glass isimproved.

In the chemically strengthened glass I of the present invention, thesurface compressive stress value (CS) is preferably 300 MPa or more. Inthe case where CS of the chemically strengthened glass is 300 MPa ormore, the glass has a good strength as a cover glass for a smart phoneand a tablet, so that the case is preferred. The bending strength ofsuch a glass is 350 MPa or more.

When a smart phone or a tablet PC is dropped, a large tensile stress isgenerated on the back surface of the cover glass by the collision withan obtuse-angle object or a rounded protruding object, and the degreethereof reaches about 350 MPa. At this time, in the case where CS is 300MPa or more, the cover glass can endure a tensile stress of about 350MPa, so that the case is preferred. CS of the chemically strengthenedglass is more preferably 350 MPa or more, further preferably 400 MPa ormore, and still further preferably 450 MPa or more.

On the other hand, an upper limit of CS of the chemically strengthenedglass I is not particularly limited but, from the viewpoint of safety atfracture, it is, for example, 2,000 MPa or less, preferably 1,500 MPa orless, more preferably 1,000 MPa or less, and further preferably 800 MPaor less.

CS of the chemically strengthened glass can be appropriately adjusted byadjusting the chemical strengthening conditions, the glass composition,and the like.

CS of the chemically strengthened glass I of the present invention isdefined as follows based on the values CS_(F) and CS_(A) resulting fromthe two kinds of measurement methods below. The same shall be applied tothe compressive stress value (CS_(x)) at a portion of x μm deep from theglass surface.

CS=CS _(F)=1.28×CS _(A)

Here, CS_(F) is a value measured by means of a surface stress meterFSM-6000 manufactured by Orihara Manufacturing Co., Ltd. and determinedby a program FsmV attached to the surface stress meter.

CS_(A) is a value measured by the following procedure using abirefringence imaging system Abrio-IM manufactured by Tokyo Instruments,Inc. As shown in FIG. 1, a cross-section of a chemically strengthenedglass having a size of 10 mm×10 mm or more and a thickness of about 0.2to 2 mm is polished into a range of 150 to 250 μm to perform thinning.As a polishing procedure, grinding is performed to an objectivethickness plus 50 μm or so by means of a #1000 diamond-electroplatedgrinding wheel, subsequently grinding is performed to an objectivethickness plus 10 μm or so by means of a #2000 diamond-electroplatedgrinding wheel, and finally mirror-finishing with cerium oxide isperformed to obtain the objective thickness. For the thus formed samplethinned to about 200 μm, using a monochromatic light of &=546 nm as alight source, measurement of phase difference (retardation) of thechemically strengthened glass is performed with a transmitted light bymeans of the birefringence imaging system, and stress is calculated byusing the obtained value and the following expression (9):

F=δ/(C×t′)  expression (9)

In the expression (9), F represents stress (MPa), δ represents phasedifference (retardation) (nm), C represents photoelastic constant (nmcm⁻¹MPa), and t′ represents thickness of a sample (cm).

In the chemically strengthened glass I of the present invention, thecompressive stress value (CS₉₀) at a portion of 90 μm deep from theglass surface is 25 MPa or more. By adjusting CS₉₀ to 25 MPa or more,the acute-angle object flaw strength can be made high. CS₉₀ ispreferably 30 MPa or more, more preferably 35 MPa or more, furtherpreferably 40 MPa or more, particularly preferably 45 MPa or more, andmost preferably 50 MPa or more.

On the other hand, an upper limit of CS₉₀ is not particularly limitedbut, from the viewpoint of safety at fracture, it is, for example, 250MPa or less, preferably 200 MPa or less, further preferably 150 MPa orless, particularly preferably 100 MPa or less, and most preferably 75MPa or less.

In the chemically strengthened glass I of the present invention, fromthe viewpoint of improving the acute-angle object flaw strength, thecompressive stress value (CS₁₀₀) at a portion of 100 μm deep from theglass surface is preferably 15 MPa or more. CS₁₀₀ is preferably 20 MPaor more, more preferably 23 MPa or more, further preferably 26 MPa ormore, particularly preferably 30 MPa or more, and most preferably 33 MPaor more.

On the other hand, an upper limit of CS₁₀₀ is not particularly limitedbut, from the viewpoint of safety at fracture, it is, for example, 200MPa or less, preferably 150 MPa or less, further preferably 100 MPa orless, particularly preferably 75 MPa or less, and most preferably 50 MPaor less.

CS₉₀ and CS₁₀₀ of the chemically strengthened glass can be appropriatelyadjusted, similarly to CS, by adjusting the chemical strengtheningconditions, the glass composition, and the like.

In the chemically strengthened glass I of the present invention,ΔCS₁₀₀₋₉₀ (unit: MPa/μm) is preferably 0.4 or more, in which ΔCS₁₀₀₋₉₀is calculated according to the following expression using CS₉₀ andCS₁₀₀.

ΔCS ₁₀₀₋₉₀=(CS ₉₀ −CS ₁₀₀)/(100−90)

By adjusting ΔCS₁₀₀₋₉₀ to 0.4 or more, the bending strength afterflawing with an acute-angle object (bending strength after flawing) canbe made high. ΔCS₁₀₀₋₉₀ is more preferably, below stepwise, 0.5 or more,0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more, 1.0 or more, 1.2 ormore, 1.4 or more, 1.6 or more, 1.8 or more, or 2.0 or more. On theother hand, an upper limit of ΔCS₁₀₀₋₉₀ is not particularly limited but,from the viewpoint of safety at fragmentation, for example, it ispreferably 4.0 or less, below stepwise, 3.0 or less, 2.0 or less, 1.8 orless, 1.6 or less, or 1.4 or less.

In the chemically strengthened glass of the present invention,ΔCS_(DOL-20) (unit: MPa/μm) is preferably 0.4 or more, in whichΔCS_(DOL-20) is calculated according to the following expression using acompressive stress value CS_(DOL-20) at a depth closer to a glasssurface side by 20 μm from DOL.

ΔCS _(DOL-20) =CS _(DOL-20)/20

By adjusting ΔCS_(DOL-20) to 0.4 or more, the bending strength afterflawing with an acute-angle object (bending strength after flawing) canbe made high. ΔCS_(DOL-20) is more preferably, below stepwise, 0.5 ormore, 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more, 1.0 or more,1.2 or more, 1.4 or more, or 1.5 or more. On the other hand, an upperlimit of ΔCS_(DOL-20) is not particularly limited but, from theviewpoint of safety at fragmentation, it is, for example, 4.0 or less,preferably 3.0 or less, more preferably 2.0 or less, further preferably1.7 or less, and typically 1.6 or less.

In the chemically strengthened glass I of the present invention, thedepth of the compressive stress layer (DOL) is preferably 100 μm ormore. In the case where DOL is 100 μm or more, resistance to crackingthat may result from the acute-angle object flawing at the time of thedrop onto sand or the like is improved. In order to enhance the strengthof the chemically strengthened glass, DOL is preferably 100 μm or moreand more preferably, below stepwise, 110 μm or more, 120 μm or more, 130μm or more, 140 μm or more, 150 μm or more, or 160 μm or more.

On the other hand, an upper limit of DOL is not particularly limitedbut, from the viewpoint of safety at fracture, it is, for example, 200μm or less, preferably 190 μm or less, further preferably 180 μm orless, and particularly preferably 150 μm or less.

DOL can be appropriately adjusted by adjusting the chemicalstrengthening conditions, the glass composition, and the like.

In the present Description, DOL is depth, from the glass surface, of aportion at which stress becomes zero in the stress profile, and is avalue measured by means of a surface stress meter FSM-6000 manufacturedby Orihara Manufacturing Co., Ltd. and analyzed by an attached programFsmV. Also, it can be measured by using a birefringence imaging systemAbrio-IM manufactured by Tokyo Instruments, Inc. with a thinned sampleas shown in FIG. 1(b).

In the chemically strengthened glass I of the present invention, thevalue of the area Sc (MPa·μm) of the compressive stress layer ispreferably 20,000 MPa·μm or more. In the case where Sc is 20,000 MPa·μmor more, a high bending strength can be maintained even in a state thatthe glass is flawed with an acute-angle object. Sc is preferably 20,000MPa·μm or more and, below stepwise, more preferably 22,000 MPa·μm ormore, 24,000 MPa·μm or more, 26,000 MPa·μm or more, 28,000 MPa·μm ormore, 30,000 MPa·μm or more, 32,000 MPa·μm or more, 34,000 MPa·μm ormore, 36,000 MPa·μm or more, or 38,000 MPa·μm or more. On the otherhand, an upper limit of Sc is not particularly limited but, from theviewpoint of safety at fragmentation, it is, for example, 50,000 MPa·μmor less and preferably 45,000 MPa·μm or less.

Sc (MPa·μm) of the chemically strengthened glass I of the presentinvention is defined as follows based on the values Sc_(F) and Sc_(A)resulting from the two kinds of measurement methods below.

Sc=Sc _(F)=1.515×Sc _(A)

Here, Sc_(F) is a value calculated by using a value measured by means ofa surface stress meter FSM-6000 manufactured by Orihara ManufacturingCo., Ltd. and analyzed by an attached program FsmV. Sc_(A) is a valueobtained by measurement using a birefringence imaging system Abrio-IMand a thinned sample, which is a method similar to the aforementionedCS_(A) measurement.

Moreover, the area St (MPa·μm) of the internal tensile layer of thechemically strengthened glass I of the present invention is defined asfollows based on the values St_(F) and St_(A) resulting from the twokinds of measurement methods below.

St=St _(F)=1.515×St _(A)

Here, St_(F) is a value calculated by using a value measured by means ofa surface stress meter FSM-6000 manufactured by Orihara ManufacturingCo., Ltd. and analyzed by an attached program FsmV. St_(A) is a valueobtained by measurement using a birefringence imaging system Abrio-IMand a thinned sample, which is a method similar to the aforementionedCS_(A) measurement. Similarly to the above, a stress profile is preparedby the two methods, St_(F) or St_(A) is calculated, and thus St can beobtained.

FIG. 2 shows a conceptual view of Sc and St. Sc and St are theoreticallyan equal value and it is preferred to calculate them so as to be0.95<Sc/St<1.05.

Further, in the chemically strengthened glass I of the presentinvention, the value Sc/t MPa) obtained by dividing the area Sc (MPa·μm)of the compressive stress layer by the sheet thickness t (μm) ispreferably 28 MPa or more. In the case where Sc/t is 28 MPa or more, theresistance to cracking that may result from the acute-angle objectflawing at the time of drop onto sand and the like is improved. Sc/t ismore preferably 30 MPa or more and further preferably, below stepwise,32 MPa or more, 34 MPa or more, 36 MPa or more, 38 MPa or more, 40 MPaor more, 42 MPa or more, 44 MPa or more, 46 MPa or more, 48 MPa or more,or 50 MPa or more. On the other hand, an upper limit of Sc/t is notparticularly limited but, from the viewpoint of safety at fragmentation,it is, for example, 60 MPa or less and preferably 55 MPa or less.

Moreover, in the chemically strengthened glass I of the presentinvention, the number of fragments generated within a size of 25 mm×25mm is 20 or less in a fracture test according to an indenter indentationtest under the condition of holding a load ranging from 5 kgf to 10 kgffor 15 seconds with a pyramidal diamond indenter in which an indenterangle of the facing angle is 60°. In the case where the number offragments (fragmentation number) in the fracture test by the indenterindentation test is 20 or less, even when the glass is fractured, a highsafety can be ensured. The fragmentation number is preferably 10 or lessand more preferably 5 or less.

Furthermore, in the chemically strengthened glass I of the presentinvention, the bending strength after flawing is preferably 150 MPa ormore. The degree of tensile stress generated on the cover glass surfaceat the time of the drop of a smart phone is about 150 MPa. Then, in thecase where the bending strength is 150 MPa or more, fracture resultingfrom the stress generated by the drop can be prevented even after theoccurrence of flawing with an acute-angle object. The bending strengthafter flawing is preferably 200 MPa or more and more preferably 250 MPaor more. As a method for flawing, there can be used an indenterindentation test where a diamond indenter (indenter angle of facingangle: 110°) is pressed or a similar method.

In the chemically strengthened glass I of the present invention, afracture stress value σa (bending strength, unit: MPa) obtained byperforming four-point bending test under conditions of an under spun of30 mm, an upper spun of 10 mm, and a crosshead speed of 0.5 mm/minuteafter the glass surface is flawed by pressing a diamond indenter(indenter angle of facing angle: 110°) thereto for 15 seconds at a loadof 0.5 kgf is preferably 150 MPa or more. σa is preferably 200 MPa ormore, more preferably 250 MPa or more, and further preferably 300 MPa ormore.

In the chemically strengthened glass I of the present invention, thefracture stress value σb (bending strength, unit: MPa) obtained byperforming four-point bending test under conditions of an under spun of30 mm, an upper spun of 10 mm, and a crosshead speed of 0.5 mm/minuteafter the glass surface is flawed by pressing a diamond indenter(indenter angle of facing angle: 110°) thereto for 15 seconds at a loadof 1 kgf is preferably 150 MPa or more. σb is preferably 200 MPa ormore, more preferably 250 MPa or more, and further preferably 300 MPa ormore.

In the chemically strengthened glass I of the present invention, thefracture stress value σc (bending strength, unit: MPa) obtained byperforming four-point bending test under conditions of an under spun of30 mm, an upper spun of 10 mm, and a crosshead speed of 0.5 mm/minuteafter the glass surface is flawed by pressing a diamond indenter(indenter angle of facing angle: 110°) thereto for 15 seconds at a loadof 2 kgf is preferably 150 MPa or more. σc is preferably 200 MPa ormore, more preferably 250 MPa or more, and further preferably 300 MPa ormore.

The following will describe the chemically strengthened glass II of thepresent invention in detail.

The chemically strengthened glass II of an embodiment is a chemicallystrengthened glass which satisfies the following expressions (1) and(2).

NM/Nh≥1.8  (1)

KM/Kh≥3  (2)

Here, NM, Nh, KM, and Kh each represents the following:

NM: a value obtained by converting a maximum value in an Na ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to Na₂O (% by weight);

Nh: a value obtained by converting a central value of sheet thickness inan Na ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight);

KM: a value obtained by converting a maximum value in a K ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to K₂O (% by weight); and

Kh: a value obtained by converting a central value of sheet thickness ina K ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to K₂O (% by weight).

Incidentally, EPMA means Electron Probe Micro Analyzer (electron probemicroanalyzer).

The value (Nh) obtained by converting a central value of sheet thicknessin an Na ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight) isa value obtained by converting Na ion concentration at a center of sheetthickness to Na₂O (% by weight) and, for example, in the case of achemically strengthened glass having a sheet thickness of 0.8 mm, it isa value obtained by converting the Na ion concentration at a position of0.4 mm from the surface to Na₂O (% by weight). Kh is to be definedsimilarly.

(NM/Nh≥1.8  (1))

As also shown in Examples to be mentioned later, according to thefindings of the present inventors, a strong correlation is presentbetween the compressive stress value at a portion of 20 to 40 μm deepfrom the glass surface and the bending strength after flawing and, ahigher compressive stress value at a portion of 20 to 40 μm deep fromthe glass surface tends to result in a higher bending strength afterflawing.

Further, as NM/Nh increases, the compressive stress value at a portionof 20 to 40 μm deep from the glass surface tends to increase. In thepresent invention, in order to achieve a sufficient bending strengthafter flawing for improving the failure rate of the cover glass, it isnecessary that NM/Nh is 1.8 or more and, below stepwise, it ispreferably 2 or more, 2.2 or more, 2.4 or more, 2.6 or more, 2.8 ormore, 3 or more, 3.2 or more, or 3.4 or more. On the other hand, anupper limit of NM/Nh is not particularly limited but, from the viewpointof safety at fragmentation, it is, for example, preferably 5 or less,more preferably 4.5 or less, and further preferably 4 or less.

(N0/Nh≥0.8  (3))

Moreover, in the chemically strengthened glass of the presentembodiment, N0/Nh that is a ratio of N0, which is a value obtained byconverting a value on an outermost surface in an Na ion concentrationprofile in a sheet thickness direction of the chemically strengthenedglass measured by EPMA to Na₂O (% by weight), to the aforementioned Nhis preferably 0.8 or more.

According to the findings of the present inventors, as N0/Nh increases,the compressive stress value at a portion of 20 to 40 μm deep from theglass surface tends to increase. In the present embodiment, in order toachieve a sufficient bending strength after flawing for improving thefailure rate of the cover glass, N0/Nh is preferably 0.8 or more and,below stepwise, it is more preferably 1 or more, 1.2 or more, 1.4 ormore, 1.6 or more, 1.8 or more, 2 or more, 2.2 or more, 2.4 or more, 2.6or more, 2.8 or more, or 3 or more. On the other hand, an upper limit ofN0/Nh is not particularly limited but, from the viewpoint of safety atfragmentation, it is, for example, preferably 5 or less, more preferably4.5 or less, and further preferably 4 or less.

(N0/NM≥0.4  (4))

Furthermore, in the chemically strengthened glass of the presentembodiment, N0/NM that is a ratio of the aforementioned N0 to theaforementioned NM is preferably 0.4 or more.

According to the findings of the present inventors, as N0/NM increases,the compressive stress value at a portion of 20 to 40 μm deep from theglass surface tends to increase. In the present embodiment, in order toachieve a sufficient bending strength after flawing for improving thefailure rate of the cover glass, N0/NM is preferably 0.4 or more and,below stepwise, it is more preferably 0.5 or more, 0.6 or more, 0.7 ormore, 0.8 or more, 0.9 or more, or 0.95 or more.

(KM/Kh≥3  (2))

In the chemically strengthened glass II of the present embodiment, alarge compressive stress is introduced into the glass surface layer byNa—K exchange where Na ions in the glass are exchanged with K ions toenhance the bending strength. From the viewpoint of improving strengthreliability of the cover glass, the bending strength is preferably 400MPa or more and, for achieving this, it is necessary that KM/Kh is 3 ormore and, below stepwise, it is more preferably 3.5 or more, 4 or more,4.5 or more, 5 or more, 5.5 or more, 6 or more, 6.5 or more, or 7 ormore.

Moreover, the chemically strengthened glass II of the other oneembodiment is a chemically strengthened glass which satisfies thefollowing expressions (5) and (6).

NM−Nh≥2.2 (wt %)  (5)

KM−Kh≥3 (wt %)  (6)

(NM−Nh≥2.2 (wt %)  (5))

As NM−Nh increases, the compressive stress value at a portion of 20 to40 μm deep from the glass surface tends to increase. In the presentembodiment, in order to achieve a sufficient bending strength afterflawing for improving the failure rate of the cover glass, it isnecessary that NM−Nh is 2.2 wt % or more and, below stepwise, it ispreferably 2.4 wt % or more, 2.6 wt % or more, 2.8 wt % or more, 3 wt %or more, 3.2 wt % or more, 3.4 wt % or more, 3.6 wt % or more, 3.8 wt %or more, 4 wt % or more, 4.2 wt % or more, 4.4 wt % or more, 4.6 wt % ormore, 4.8 wt % or more, or 5 wt % or more. On the other hand, an upperlimit of NM−Nh is not particularly limited but, from the viewpoint ofsafety at fragmentation, it is, for example, preferably 7 wt % or less,more preferably 6.5 wt % or less, and further preferably 6 wt % or less.

(N0−Nh≥−0.4 (wt %)  (7))

Moreover, in the chemically strengthened glass II of the presentembodiment, N0−Nh that is a difference between N0, which is a valueobtained by converting a value on an outermost surface in an Na ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to Na₂O (% by weight), and theaforementioned Nh is preferably −0.4 wt % or more.

According to the findings of the present inventors, as N0−Nh increases,the compressive stress value at a portion of 20 to 40 μm deep from theglass surface tends to increase. In the present embodiment, in order toachieve a sufficient bending strength after flawing for improving thefailure rate of the cover glass, N0−Nh is preferably 0.4 wt % or moreand, below stepwise, it is more preferably 0 wt % or more, 0.5 wt % ormore, 1 wt % or more, 1.5 wt % or more, 2 wt % or more, 2.5 wt % ormore, 3 wt % or more, 3.5 wt % or more, 4 wt % or more, 4.5 wt % ormore, or 5 wt % or more. On the other hand, an upper limit of N0−Nh isnot particularly limited but, from the viewpoint of safety atfragmentation, it is, for example, preferably 7 wt % or less, morepreferably 6.5 wt % or less, and further preferably 6 wt % or less.

(N0−NM≥−3.5 (wt %)  (8))

Furthermore, in the chemically strengthened glass II of the presentembodiment, N0−NM that is a difference between the aforementioned N0 andthe aforementioned NM is preferably −3.5 wt % or more.

According to the findings of the present inventors, as N0−NM increases,the compressive stress value at a portion of 20 to 40 μm deep from theglass surface tends to increase. In the present embodiment, in order toachieve a sufficient bending strength after flawing for improving thefailure rate of the cover glass, N0−NM is preferably −3.5 wt % or moreand, below stepwise, it is more preferably −3 wt % or more, −2.5 wt % ormore, −2 wt % or more, −1.5 wt % or more, −1 wt % or more, −0.5 wt % ormore, −0.25 wt % or more, or −0.1 wt % or more.

(KM−Kh≥3 (wt %)  (6))

In the chemically strengthened glass II of the present embodiment, alarge compressive stress is introduced into the glass surface layer toenhance the bending strength by Na—K exchange where Na ions in the glassare exchanged with K ions. From the viewpoint of improving strengthreliability of the cover glass, the bending strength is preferably 400MPa or more and, for achieving this, it is necessary that KM−Kh is 3 wt% or more and, below stepwise, it is preferably 3.5 wt % or more, 4 wt %or more, 4.5 wt % or more, 5 wt % or more, 5.5 wt % or more, 6 wt % ormore, 6.5 wt % or more, 7 wt % or more, 7.5 wt % or more, 8 wt % ormore, 8.5 wt % or more, 9 wt % or more, 9.5 wt % or more, or 10 wt % ormore.

In the chemically strengthened glass II of the present invention, thesurface compressive stress (CS) is preferably 400 MPa or more. Forimproving strength reliability of the cover glass, the bending strengthof the glass is preferably 400 MPa or more. In the case where CS of thechemically strengthened glass is 400 MPa or more, the bending strengthof the glass is 400 MPa or more. CS of the chemically strengthened glassis more preferably 500 MPa or more and further preferably 600 MPa ormore.

On the other hand, an upper limit of CS of the chemically strengthenedglass II is not particularly limited but, from the viewpoint of safetyat fracture, it is, for example, 2,000 MPa or less, preferably 1,500 MPaor less, more preferably 1,000 MPa or less, and further preferably 800MPa or less.

CS of the chemically strengthened glass can be appropriately adjusted byadjusting the chemical strengthening conditions, the glass composition,and the like.

CS of the chemically strengthened glass II of the present invention isdefined as follows based on the values CS_(F) and CS_(A) resulting fromthe two kinds of measurement methods below. The same shall be applied tothe compressive stress value (CS_(x)) at a portion of x μm deep from theglass surface.

CS=CS _(F)=1.28×CS _(A)

Here, CS_(F) is a value measured by means of a surface stress meterFSM-6000 manufactured by Orihara Manufacturing Co., Ltd. and determinedby a program FsmV attached to the surface stress meter.

CS_(A) is a value measured by the aforementioned procedure using abirefringence imaging system Abrio-IM manufactured by Tokyo Instruments,Inc.

In the chemically strengthened glass II of the present invention, thebending strength after flawing is preferably 200 MPa or more. When asmart phone or a tablet PC is dropped, tensile stress is generated onthe cover glass surface and the degree reaches about 200 MPa. Since aflaw is generated on the cover glass surface by the collision with anacute-angle object or the like, the failure rate of the cover glass canbe improved in the case where the bending strength is 200 MPa or moreeven in the state that a flaw is present.

Here, the bending strength after flawing means a fracture stress value(bending strength, unit: MPa) obtained by performing four-point bendingtest under conditions of an under spun of 30 mm, an upper spun of 10 mm,and a crosshead speed of 0.5 mm/minute after the glass surface is flawedby pressing a diamond indenter (indenter angle of facing angle: 110°)thereto for 15 seconds at a load of 0.5 kgf or 1 kgf.

In the chemically strengthened glass II of the present invention, thecompressive stress value (CS₂₀) at a portion of 20 μm deep from theglass surface is preferably 60 MPa or more. In the case where CS₂₀ is 60MPa or more, the bending test strength after flawing can be made 200 MPaor more and the failure rate of the cover glass is improved. CS₂₀ ismore preferably 80 MPa or more and, below stepwise, it is furtherpreferably 100 MPa or more, 120 MPa or more, 140 MPa or more, 160 MPa ormore, 180 MPa or more, 200 MPa or more, 220 MPa or more, 240 MPa ormore, 260 MPa or more, 280 MPa or more, or 300 MPa or more.

On the other hand, an upper limit of CS₂₀ is not particularly limitedbut, from the viewpoint of safety at fragmentation, it is, for example,500 MPa or less, preferably 400 MPa or less, further preferably 350 MPaor less, and particularly preferably 320 MPa or less.

In the chemically strengthened glass II of the present invention, thecompressive stress value (CS₄₀) at a portion of 40 μm deep from theglass surface is preferably 60 MPa or more. In the case where CS₄₀ is 60MPa or more, the bending test strength after flawing can be made 200 MPaor more and the failure rate of the cover glass is improved. CS₄₀ ismore preferably, below stepwise, 70 MPa or more, 80 MPa or more, 90 MPaor more, 100 MPa or more, 110 MPa or more, 120 MPa or more, 130 MPa ormore, 150 MPa or more, 160 MPa or more, 170 MPa or more, or 180 MPa ormore.

On the other hand, an upper limit of CS₄₀ is not particularly limitedbut, from the viewpoint of safety at fragmentation, it is, for example,300 MPa or less, preferably 250 MPa or less, and further preferably 200MPa or less.

CS₂₀ and CS₄₀ of the chemically strengthened glass can be appropriatelyadjusted similarly to CS, by adjusting the chemical strengtheningconditions, the glass composition, and the like.

Moreover, in the chemically strengthened glass II of the presentinvention, the depth of the compressive stress layer (DOL) is preferably100 μm or more. In the case where DOL is 100 μm or more, a remarkablestrength improvement becomes possible against fracture caused by flawingwith an acute-angle object at the time of drop onto sand or the like.DOL is more preferably 110 μm or more, further preferably 130 μm ormore, and particularly preferably 150 μm or more.

On the other hand, an upper limit of DOL is not particularly limitedbut, from the viewpoint of safety at fracture, it is, for example, 200μm or less, preferably 180 μm or less, and further preferably 160 μm orless.

DOL can be appropriately adjusted by adjusting the chemicalstrengthening conditions, the glass composition, and the like.

DOL of the chemically strengthened glass II of the present invention isdepth, from the glass surface, of a portion at which stress becomes zeroin the stress profile and is a value measured by means of a surfacestress meter FSM-6000 manufactured by Orihara Manufacturing Co., Ltd.and analyzed by an attached program FsmV. Also, it can be measured byusing a birefringence imaging system Abrio-IM manufactured by TokyoInstruments, Inc. with a thinned sample as shown in FIG. 1(b).

Moreover, in the chemically strengthened glass II of the presentinvention, the number of fragments generated within a size of 25 mm×25mm is 20 or less in a fracture test according to an indenter indentationtest under the condition of holding a load ranging from 5 kgf to 10 kgffor 15 seconds with a pyramidal diamond indenter in which an indenterangle of the facing angle is 60°. In the case where the number offragments (fragmentation number) in the fracture test by the indenterindentation test is 20 or less, even when the glass is fractured, a highsafety can be ensured. The fragmentation number is preferably 30 or lessand more preferably 40 or less.

Subsequently, the matrix composition of the chemically strengthenedglass in the present invention will be described.

In the present Description, the matrix composition of the chemicallystrengthened glass means the composition of the glass before chemicalstrengthening (hereinafter sometimes referred to as matrix glass orglass for chemical strengthening). Here, it is considered that a portionhaving tensile stress (hereinafter also referred to as tensile stressportion) in the chemically strengthened glass is considered to be aportion which is not ion-exchanged. Accordingly, the tensile stressportion in the chemically strengthened glass has the same composition asthe matrix glass and the composition of the tensile stress portion canbe regarded as the matrix composition.

The following will describe suitable content of each component that maybe contained in the matrix composition of the chemically strengthenedglass. The content of each component is indicated in terms of molarpercentage on the basis of oxides unless otherwise stated.

The composition of the glass can be simply determined bysemi-qualitative analysis by an X-ray fluorescent method but, moreprecisely, can be measured by a wet analyzing method such as ICPemission analysis.

As the composition for the glass for chemical strengthening of thepresent invention (matrix composition of the chemically strengthenedglass of the present invention), for example, there may be mentioned onecontaining from 50 to 80% of SiO₂, from 1 to 30% of Al₂O₃, from 0 to 5%of B₂O₃, from 0 to 4% of P₂O₅, from 3 to 20% of Li₂O, from 0 to 8% ofNa₂O, from 0 to 10% of K₂O, from 3 to 20% of MgO, from 0 to 20% of CaO,from 0 to 20% of SrO, from 0 to 15% of BaO, from 0 to 10% of ZnO, from 0to 1% of TiO₂, and from 0 to 8% of ZrO₂.

For example, there may be mentioned a glass containing from 63 to 80% ofSiO₂, from 7 to 30% of Al₂O₃, from 0 to 5% of B₂O₃, from 0 to 4% ofP₂O₅, from 5 to 15% of Li₂O, from 4 to 8% of Na₂O, from 0 to 2% of K₂O,from 3 to 10% of MgO, from 0 to 5% of CaO, from 0 to 20% of SrO, from 0to 15% of BaO, from 0 to 10% of ZnO, from 0 to 1% of TiO₂, and from 0 to8% of ZrO₂, and not containing Ta₂O₅, Gd₂O₃, As₂O₃, and Sb₂O₃.

SiO₂ is a component that constitutes the network of the glass. Also, itis a component that enhances chemical durability and is a component thatreduces the generation of crack(s) when the glass surface is flawed(indented), and the content of SiO₂ is preferably 50% or more. Thecontent of SiO₂ is more preferably, below stepwise, 54% or more, 58% ormore, 60% or more, 63% or more, 66% or more, or 68% or more. On theother hand, in the case where the content of SiO₂ exceeds 80%,meltability remarkably decreases. The content of SiO₂ is 80% or less,more preferably 78% or less, further preferably 76% or less,particularly preferably 74% or less, and most preferably 72% or less.

Al₂O₃ is a component that improves the fragmentation property of thechemically strengthened glass. Here, high fragmentation property of aglass means that the number of fragments at the time of breakage of theglass is small. A glass having a high fragmentation property can be saidto have a high safety, since fragments hardly scatter at the time offracture. Moreover, since Al₂O₃ is a component effective for improvingthe ion exchange performance at the time of chemical strengthening andincreasing the surface compressive stress after strengthening, thecontent of Al₂O₃ is preferably 1% or more. The content of Al₂O₃ is morepreferably, below stepwise, 3% or more, 5% or more, 7% or more, 8% ormore, 9% or more, 10% or more, 11% or more, 12% or more, or 13% or more.On the other hand, in the case where the content of Al₂O₃ exceeds 30%,acid resistance of the glass decreases or devitrification temperaturebecomes high. Also, viscosity of glass increases and meltabilitydecreases. The content of Al₂O₃ is preferably 30% or less, morepreferably 25% or less, further preferably 20% or less, particularlypreferably 18% or less, and most preferably 15% or less. On the otherhand, in the case where the content of Al₂O₃ is large, the temperatureat glass melting increases and productivity decreases. In the case ofconsidering the glass productivity, the content of Al₂O₃ is preferably11% or less, below stepwise, 10% or less, 9% or less, 8% or less, or 7%or less.

B₂O₃ is a component that improves chipping resistance of the glass forchemical strengthening or chemically strengthened glass and improves themeltability of the glass. Although B₂O₃ is not essential, the content inthe case of containing B₂O₃ is, for improving the meltability,preferably 0.5% or more, more preferably 1% or more, and furtherpreferably 2% or more. On the other hand, in the case where the contentof B₂O₃ exceeds 5%, striae may be generated at the time of melting andthus quality of the glass for chemical strengthening is prone todecrease. The content of B₂O₃ is more preferably 4% or less, furtherpreferably 3% or less, and particularly preferably 1% or less. In orderto enhance the acid resistance, B₂O₃ is preferably not contained.

P₂O₅ is a component that improves ion exchange performance and chippingresistance. P₂O₅ may not be contained but the content in the case ofcontaining P₂O₅ is preferably 0.5% or more, more preferably 1% or more,and further preferably 2% or more. On the other hand, in the case wherethe content of P₂O₅ exceeds 4%, the fragmentation property of the glassremarkably decreases and the acid resistance remarkably decreases. Thecontent of P₂O₅ is preferably 4% or less, more preferably 3% or less,further preferably 2% or less, and particularly preferably 1% or less.In order to enhance the acid resistance, P₂O₅ is preferably notcontained.

Li₂O is a component that forms surface compressive stress by ionexchange and is a component that improves the fragmentation property ofthe chemically strengthened glass.

In the case where Li ions at the glass surface are exchanged with Naions to perform a chemical strengthening treatment so that the aboveCS₄₀ becomes 60 MPa or more, the content of Li₂O is preferably 3% ormore, more preferably 4% or more, further preferably 5% or more,particularly preferably 6% or more, and typically 7% or more. On theother hand, in the case where the content of Li₂O exceeds 20%, the acidresistance of the glass remarkably decreases. The content of Li₂O ispreferably 20% or less, more preferably 18% or less, further preferably16% or less, particularly preferably 15% or less, and most preferably13% or less.

On the other hand, in the case where Na ions at the glass surface areexchanged with K ions to perform a chemical strengthening treatment sothat the above CS₄₀ becomes 60 MPa or more, for the content of Li₂O morethan 3%, the degree of the compressive stress decreases and it becomesdifficult to achieve CS₄₀ of 60 MPa or more. In this case, the contentof Li₂O is preferably 3% or less, more preferably 2% or less, furtherpreferably 1% or less, particularly preferably 0.5% or less, and mostpreferably, Li₂O is substantially not contained.

In the present Description, the phrase “substantially not contained”means that it is not contained except unavoidable impurities containedin raw materials and the like, i.e., it is not intentionally contained.Specifically, it indicates that the content in the glass composition isless than 0.1% by mol.

Na₂O is a component that forms a surface compressive stress layerthrough ion exchange and enhances the meltability of the glass.

In the case where Li ions at the glass surface are exchanged with Naions to perform a chemical strengthening treatment so that the aboveCS₄₀ becomes 60 MPa or more, Na₂O may not be contained but, in the caseof regarding the meltability of the glass as important, it may becontained. The content in the case of containing Na₂O is preferably 1%or more. The content of Na₂O is more preferably 2% or more and furtherpreferably 3% or more. On the other hand, in the case where the contentof Na₂O exceeds 8% or more, the surface compressive stress formed by theion exchange remarkably decreases. The content of Na₂O is preferably 8%or less, more preferably 7% or less, further preferably 6% or less,particularly preferably 5% or less, and most preferably 4% or less.

On the other hand, in the case where Na ions at the glass surface areexchanged with K ions to perform a chemical strengthening treatment sothat the above CS₄₀ becomes 60 MPa or more, Na is essential and thecontent thereof is 5% or more. The content of Na₂O is preferably 5% ormore, more preferably 7% or more, further preferably 9% or more,particularly preferably 11% or more, and most preferably 12% or more. Onthe other hand, in the case where the content of Na₂O exceeds 20%, theacid resistance of the glass remarkably decreases. The content of Na₂Ois preferably 20% or less, more preferably 18% or less, furtherpreferably 16% or less, particularly preferably 15% or less, and mostpreferably 14% or less.

In the case where Li ions and Na ions at the glass surface aresimultaneously ion-exchanged with Na ions and K ions, respectively, by amethod of immersion in a mixed molten salt of potassium nitrate andsodium nitrate or a similar method, the content of Na₂O is preferably10% or less, more preferably 9% or less, further preferably 7% or less,particularly preferably 6% or less, and most preferably 5% or less.Moreover, the content of Na₂O is preferably 2% or more, more preferably3% or more, and further preferably 4% or more.

K₂O may be contained for enhancing the ion exchange performance and thelike. The content in the case of containing K₂O is preferably 0.5% ormore, more preferably 1% or more, further preferably 2% or more, andparticularly preferably 3% or more. On the other hand, in the case wherethe content of K₂O exceeds 10%, the fragmentation property of thechemically strengthened glass decreases, so that the content of K₂O ispreferably 10% or less. The content of K₂O is more preferably 8% orless, further preferably 6% or less, particularly preferably 4% or less,and most preferably 2% or less.

MgO is a component that increases the surface compressive stress of thechemically strengthened glass and is a component that improves thefragmentation property, and thus, it is preferably contained. Thecontent in the case of containing MgO is preferably 3% or more, morepreferably, below stepwise, 4% or more, 5% or more, 6% or more, 7% ormore, or 8% or more. On the other hand, in the case where the content ofMgO exceeds 20%, the glass for chemical strengthening is prone to bedevitrified at the time of melting. The content of MgO is preferably 20%or less and more preferably, below stepwise, 18% or less, 15% or less,14% or less, 13% or less, 12% or less, 11% or less, or 10% or less.

CaO is a component that enhances the meltability of the glass forchemical strengthening and is a component that improves thefragmentation property, and it may be contained. The content in the caseof containing CaO is preferably 0.5% or more, more preferably 1% ormore, further preferably 2% or more, particularly preferably 3% or more,and most preferably 5% or more. On the other hand, in the case where thecontent of CaO exceeds 20%, the ion exchange performance remarkablydecreases, so that the content is preferably 20% or less. The content ofCaO is more preferably 14% or less and further preferably 10% or less,8% or less, 6% or less, 3% or less, and 1% or less.

SrO is a component that enhances the meltability of the glass forchemical strengthening and a component that improves the fragmentationproperty of the chemically strengthened glass, and it may be contained.The content in the case of containing SrO is preferably 0.5% or more,more preferably 1% or more, further preferably 2% or more, particularlypreferably 3% or more, and most preferably 5% or more. On the otherhand, in the case where the content of SrO exceeds 20%, the ion exchangeperformance remarkably decreases, so that the content is preferably 20%or less. The content of SrO is more preferably 14% or less and furtherpreferably 10% or less, 8% or less, 6% or less, 3% or less, and 1% orless.

BaO is a component that enhances the meltability of the glass forchemical strengthening and a component that improves the fragmentationproperty of the chemically strengthened glass, and it may be contained.The content in the case of containing BaO is preferably 0.5% or more,more preferably 1% or more, further preferably 2% or more, particularlypreferably 3% or more, and most preferably 5% or more. On the otherhand, in the case where the content of BaO exceeds 15%, the ion exchangeperformance remarkably decreases. The content of BaO is preferably 15%or less and more preferably, below stepwise, 10% or less, 8% or less, 6%or less, 3% or less, or 1% or less.

ZnO is a component that enhances the meltability of the glass and may becontained. The content in the case of containing ZnO is preferably 0.25%or more and more preferably 0.5% or more. On the other hand, in the casewhere the content of ZnO exceeds 10%, the weathering resistance of theglass remarkably decreases. The content of ZnO is preferably 10% orless, more preferably 7% or less, further preferably 5% or less,particularly preferably 2% or less, and most preferably 1% or less.

TiO₂ is a component that improves the fragmentation property of thechemically strengthened glass and may be contained. The content in thecase of containing TiO₂ is preferably 0.1% or more, more preferably0.15% or more, and further preferably 0.2% or more. On the other hand,in the case where the content of TiO₂ exceeds 5%, the glass is prone tobe devitrified at the time of melting and there is a concern of adecrease in the quality of the chemically strengthened glass. Thecontent of TiO₂ is preferably 1% or less, more preferably 0.5% or less,and further preferably 0.25% or less.

ZrO₂ is a component that increases the surface compressive stress by ionexchange and has an effect of improving the fragmentation property ofthe glass for chemical strengthening, and it may be contained. Thecontent in the case of containing ZrO₂ is preferably 0.5% or more andmore preferably 1% or more. On the other hand, in the case where thecontent of ZrO₂ exceeds 8%, the glass is prone to be devitrified at thetime of melting and there is a concern of a decrease in the quality ofthe chemically strengthened glass. The content of ZrO₂ is preferably 8%or less, more preferably 6% or less, further preferably 4% or less,particularly preferably 2% or less, and most preferably 1.2% or less.

Y₂O₃, La₂O₃, and Nb₂O₅ are components that improve the fragmentationproperty of the chemically strengthened glass and may be contained. Thecontent of each one in the case of containing these components ispreferably 0.5% or more, more preferably 1% or more, further preferably1.5% or more, particularly preferably 2% or more, and most preferably2.5% or more. On the other hand, in the case where the content of eachof Y₂O₃, La₂O₃, and Nb₂O₅ exceeds 8%, the glass is prone to bedevitrified at the time of melting and there is a concern of a decreasein the quality of the chemically strengthened glass. The content of eachof Y₂O₃, La₂O₃, and Nb₂O₅ is preferably 8% or less, more preferably 6%or less, further preferably 5% or less, particularly preferably 4% orless, and most preferably 3% or less.

Ta₂O₅ and Gd₂O₃ may be contained in a small amount for improving thefragmentation property of the chemically strengthened glass but, sincethe refractive index and the reflectance increase, the content ispreferably 1% or less and more preferably 0.5% or less, and furtherpreferably, they are not contained.

Furthermore, for the use of the colored glass, a coloring component maybe added within the range where the achievement of the desired chemicalstrengthening properties is not inhibited. Examples of the coloringcomponent include Co₃O₄, MnO₂, Fe₂O₃, NiO, CuO, Cr₂O₃, V₂O₅, Bi₂O₃,SeO₂, TiO₂, CeO₂, Er₂O₃, Nd₂O₃, and the like as suitable ones.

The content of the coloring component is preferably in the range of 7%or less in total in terms of molar percentage on the basis of oxides. Inthe case where the content exceeds 7%, the glass is prone to bedevitrified, so that the case is not desirable. The content ispreferably 5% or less, more preferably 3% or less, and furtherpreferably 1% or less. In the case where the visible ray transmittanceof the glass takes preference, it is preferred that these components aresubstantially not contained.

As a refining agent at the time of melting the glass, SO₃, a chloride, afluoride, or the like may be appropriately contained. As₂O₃ ispreferably not contained. In the case of containing Sb₂O₃, the contentis preferably 0.3% or less and more preferably 0.1% or less, and mostpreferably, it is not contained.

Also, the chemically strengthened glass of the present inventionpreferably has at least one kind selected from the group consisting ofsodium ions, silver ions, potassium ions, cesium ions, and rubidium ionsat the surface. Thereby, the compressive stress is induced at thesurface and the glass is highly strengthened. Furthermore, in the casewhere the silver ions are present at the surface, an antibacterialproperty can be imparted.

In the present invention, it is preferred to select a matrix compositionof the chemically strengthened glass so that DOL is 50 μm or more in thecase where an ion exchange treatment is performed for 1 hour with amolten salt at 400° C. composed of KNO₃, NaNO₃, or a mixed salt of KNO₃and NaNO₃ on a glass sheet having a thickness of 1 mm which has thematrix composition of the chemically strengthened glass and has beengradually cooled under the following conditions. Here, the gradualcooling is performed from a temperature T° C., which is 30° C. to 50° C.higher than the glass transition temperature, to (T−300)° C. at acooling rate of 0.5° C./minute.

Furthermore, in the present invention, it is preferred to select amatrix composition of the chemically strengthened glass so that DOL is70 μm or more in the case where an ion exchange treatment is performedfor 1 hour with a molten salt at 425° C. composed of KNO₃, NaNO₃, or amixed salt of KNO₃ and NaNO₃ on a glass sheet having a thickness of 1 mmwhich has the matrix composition of the chemically strengthened glassand has been gradually cooled under the following conditions. Here, thegradual cooling is performed from a temperature T° C., which is 30° C.to 50° C. higher than the glass transition temperature, to (T−300)° C.at a cooling rate of 0.5° C./minute.

In the case of such a matrix composition, the ion exchange rate is highand chemical strengthening can be performed for a short period of time.

In the case where the chemically strengthened glass of the presentinvention is a sheet-form (glass sheet), the sheet thickness (t) thereofis not particularly limited but is, from the viewpoint of enabling aremarkable improvement of strength by chemical strengthening, forexample, 2 mm or less, preferably 1.5 mm or less, more preferably 1 mmor less, further preferably 0.9 mm or less, particularly preferably 0.8mm or less, and most preferably 0.7 mm or less. Further, the sheetthickness is, form the viewpoint of obtaining an effect of a sufficientimprovement of strength by chemical strengthening, for example, 0.1 mmor more, preferably 0.2 mm or more, more preferably 0.4 mm or more, andfurther preferably 0.5 mm or more.

The chemically strengthened glass of the present invention may have ashape other than the sheet-form, e.g., a fringed shape having adifferent thickness at outer periphery, depending on the products, uses,and the like to which the glass is applied. Moreover, the above glasssheet has two main surfaces and end surfaces that neighbors them to formthe thickness and the two main surfaces may form flat faces that areparallel to each other. However, the configuration of the glass sheet isnot limited thereto and, for example, the two main surfaces may not beparallel to each other or all or a part of one or both of the two mainsurfaces may be curved. More specifically, the glass sheet may be, forexample, a warpage-free flat shape glass sheet or may be a curved glasssheet having a curved surface.

In the present invention, it is preferred that the Young's modulus ofthe glass for chemical strengthening is 70 GPa or more and also adifference between the compressive stress value (CS₀) on the outermostsurface of the chemically strengthened glass and the compressive stressvalue (CS₁) at a portion of 1 μm deep from the glass surface is 50 MPaor less. In the case where they are controlled as such, warpage isdifficult to occur when a polishing treatment of the glass surface isperformed after the chemical strengthening treatment, so that the caseis preferred.

The Young's modulus of the glass for chemical strengthening is morepreferably 74 GPa or more, particularly preferably 78 GPa or more, andfurther preferably 82 GPa or more. An upper limit of the Young's modulusis not particularly limited but is, for example, 90 GPa or less andpreferably 88 GPa or less. The Young's modulus can be measured, forexample, by an ultrasonic pulse method.

The difference between CS₀ and CS₁ is preferably 50 MPa or less, morepreferably 40 MPa or less, and further preferably 30 MPa or less.

Furthermore, CS₀ is preferably 300 MPa or more, more preferably 350 MPaor more, and further preferably 400 MPa or more. On the other hand, anupper limit of CS₀ is not particularly limited but is, for example,1,200 MPa or less, preferably 1,000 MPa or less, and further preferably800 MPa or less.

In addition, CS₁ is preferably 250 MPa or more, more preferably 300 MPaor more, and further preferably 350 MPa or more. On the other hand, anupper limit of CSI is not particularly limited but is, for example,1,150 MPa or less, preferably 1,100 MPa or less, and further preferably1,050 MPa or less.

The chemically strengthened glass of the present invention can bemanufactured, for example, as follows. The following manufacturingmethod is an example in the case of manufacturing a sheet-formchemically strengthened glass.

First, a glass to be subjected to the aforementioned chemicalstrengthening treatment (glass for chemical strengthening) is prepared.For example, raw materials of respective components of the glass areblended and heated to melt in a glass melting furnace. Subsequently, theglass is homogenized by bubbling, stirring, addition of a refiningagent, or the like, and then formed into a glass sheet having apredetermined thickness according to a conventionally known method, andthereafter gradually cooled.

Examples of the glass forming method includes, a float method, a pressmethod, a fusion method, and a down-draw method. In particular, a floatmethod suitable for mass production is preferred. Other continuousforming methods than a float method, that is, a fusion method and adown-draw method are also preferred.

Subsequently, the formed glass is subjected to grinding and polishingtreatments as needed to form a glass substrate. In the case where theglass substrate is cut into a predetermined shape and size or chamferingof the glass substrate is performed, when the cutting and chamfering ofthe glass substrate are performed before the chemical strengtheningtreatment to be mentioned below, a compressive stress layer is formedalso on the end surface by the subsequent chemical strengtheningtreatment, so that the case is preferred.

Then, after the formed glass substrate is subjected to the chemicalstrengthening treatment, the chemically strengthened glass of thepresent invention can be manufactured by washing and drying.

In the chemical strengthening treatment, by bringing the glass intocontact with a molten liquid of a metal salt (e.g., potassium nitrate)containing metal ions having a large ionic radius (typically, Na ions orK ions) through immersion or the like, metal ions having a small ionicradius (typically, Na ions or Li ions) in the glass are substituted withmetal ions having a large ionic radius.

The chemical strengthening treatment (ion exchange treatment) is notparticularly limited but, for example, can be performed by immersing theglass in a molten salt heated to 360 to 600° C. for 0.1 to 500 hours.The heating temperature of the molten salt is preferably from 375 to500° C. and the immersing time of the glass in the molten salt ispreferably from 0.3 to 200 hours.

As the molten salt for performing the chemical strengthening treatment,there may be mentioned nitrate salts, sulfate salts, carbonate salts,chlorides, and the like. Of these, as the nitrate salts, there may bementioned lithium nitrate, sodium nitrate, potassium nitrate, cesiumnitrate, silver nitrate, and the like. As the sulfate salts, there maybe mentioned lithium sulfate, sodium sulfate, potassium sulfate, cesiumsulfate, silver sulfate, and the like. As the carbonate salts, there maybe mentioned lithium carbonate, sodium carbonate, potassium carbonate,and the like. As the chlorides, there may be mentioned lithium chloride,sodium chloride, potassium chloride, cesium chloride, silver chloride,and the like. These molten salts may be used solely or a plurality ofthem may be used in combination.

In the present invention, the treatment conditions for the chemicalstrengthening treatment are not particularly limited and it issufficient to select appropriate conditions considering the propertiesand compositions of the glass, the kind of the molten salt, the chemicalstrengthening properties such as the surface compressive stress (CS) andthe depth of the compressive stress layer (DOL) which are desired forthe finally obtained chemically strengthened glass, and the like.

Moreover, in the present invention, the chemical strengthening treatmentmay be performed only once or plural times of chemical strengtheningtreatments (multistage strengthening) may be performed under two or moredifferent conditions. Here, for example, a chemical strengtheningtreatment is performed under a condition where CS becomes relatively lowas a first-stage chemical strengthening treatment, and then, a chemicalstrengthening treatment is performed under a condition where CS becomesrelatively high as a second-stage chemical strengthening treatment.Thus, an integrated value of compressive stress generated in thecompressive stress layer can be suppressed low while increasing CS onthe outermost surface of the chemically strengthened glass and, as aresult, internal tensile stress (CT) can be suppressed low.

Especially, in order to perform the chemical strengthening treatment ata high ion exchange rate, it is preferred to exchange Li ions in theglass with Na ions (Li—Na exchange).

As the method for manufacturing the chemically strengthened glass II ofthe present invention, from the viewpoint of improving the bendingstrength after flawing, there is preferred a method for manufacturingthe chemically strengthened glass comprising performing at least twostages of ion exchange treatments, in which KNO₃ concentration in themolten salt to be used in the first-stage ion exchange treatment is 60%by weight or more and NaNO₃ concentration in the molten salt to be usedin the second-stage ion exchange treatment is 5% by weight or more.

From the viewpoint of improving the bending strength after flawing, theKNO₃ concentration in the molten salt to be used in the first-stage ionexchange treatment is preferably 60% by weight or more and morepreferably, below stepwise, 70% by weight or more, 80% by weight ormore, or 90% by weight or more. Also, an upper limit thereof is notparticularly limited and may be, for example, 100% by weight, that is,the salt may be a molten salt composed of KNO₃ only. Here, according tothe first-stage ion exchange treatment, Na—K exchange where Na ions inthe glass are exchanged with K ions is mainly performed.

Also, from the viewpoint of improving the bending strength afterflawing, the NaNO₃ concentration in the molten salt to be used in thesecond-stage ion exchange treatment is preferably 5% by weight or moreand more preferably, below stepwise, 10% by weight or more, 20% byweight or more, 30% by weight or more, 40% by weight or more, 50% byweight or more, 60% by weight or more, 70% by weight or more, 80% byweight or more, or 90% by weight or more. Also, an upper limit thereofis not particularly limited and may be, for example, 100% by weight,that is, the salt may be a molten salt composed of NaNO₃ only. Here,according to the second-stage ion exchange treatment, at least, Li—Naexchange where Li ions in the glass are exchanged with Na ions isperformed.

The chemically strengthened glass of the present invention isparticularly useful as a cover glass to be used in, such as mobiledevices such as mobile phones, smart phones, personal digital assistant(PDA), and tablet terminals. Furthermore, it is also useful, not forpurpose of carrying, as a cover glass for display devices such astelevision (TV), personal computers (PC) and touch panels, a buildingmaterial for walls of elevators, walls (whole face displays) ofarchitectures such as houses and buildings, window panes, and the like,a table top, an interior of automobiles, aircraft and the like, and acover glass thereof.

Also it is useful in uses as non-tabular housings having curved shape bybending or forming.

Examples

The following will describe the present invention with reference toExamples but the present invention is not limited thereby. With regardto the measurement results in Tables, a blank column indicates that thedata is not measured.

(Production of Chemically Strengthened Glass)

Glasses of Examples 1 to 15 shown in Table 2 were produced as follows.

First, glass sheets of glass 1, glass 3, glass 4, glass 6, and glass 7each having a composition shown in Table 1 in terms of molar percentageon the basis of oxides were produced by platinum crucible melting. Glassmaterials that are generally used, such as oxides, hydroxides,carbonates, and nitrates were suitably selected and were weighed so asto be 1,000 g as the resultant glass. Next, the mixed material was putin a platinum crucible, set in an electric resistance heating furnace at1500 to 1700° C. and melted therein for about 3 hours, to be defoamedand homogenized. The resultant molten glass was cast into a mold, kepttherein at a temperature of glass transition temperature +50° C. for 1hour, and then cooled to room temperature at a rate of 0.5° C./minute,to obtain a glass block. The resultant glass block was cut and ground,and finally both surfaces thereof were mirror-finished to obtain asheet-form glass having a desired shape. The sheet thickness t (mm) isshown in Table 2.

Glass sheets having a composition of each of glass 2 and glass 5 shownin Table 1 in terms of molar percentage on the basis of oxides wereproduced in a float furnace. Glass materials that are generally used,such as oxides, hydroxides, carbonates, and nitrates were suitablyselected and melted in the melting furnace and forming was performed bya float method so that the sheet thickness was from 1.1 to 1.3 mmt. Theresultant sheet glass was cut and ground, and finally both surfacesthereof were mirror-finished to obtain a sheet-form glass having adesired shape. The thickness t (mm) is shown in Table 2.

TABLE 1 (mol %) Glass 1 Glass 2 Glass 3 Glass 4 Glass 5 Glass 6 Glass 7SiO₂ 70 64.5 56.1 68 67.6 69 70 Al₂O₃ 10 12 17.2 10 10.1 9 7.5 B₂O₃ 0P₂O₅ 6.8 Li₂O 10 12.8 9.5 8 Na₂O 3 5.5 16.8 8 14.2 4.5 5.3 K₂O 1 3.4 0.30.1 1 1 MgO 5 0 2.7 14 8 6 7 CaO 0 0.1 0.2 SrO 0 BaO 0 ZnO TiO₂ 0.040.04 ZrO₂ 1 1.8 0.04 1 1 Sum 100 100 99.9 100 100 100 100 Density(g/cm³) 2.42 2.40 2.45 2.44 2.44 2.44 E (GPa) 84.2 68.9 80.9 72.0 84.182.6 CS (μm) at strengthening 344 286 of 1 mmt NaNO₃ 400° C. 1 hour DOL(μm) at 77 58 strengthening of 1 mmt NaNO₃ 400° C. 1 hour

Subsequently, a chemical strengthening treatment was performed on glass1 to glass 4 under conditions described in Table 2, thereby obtainingchemically strengthened glasses of Example 1 to Example 15.

A chemical strengthening treatment was performed on glass 6 and glass 7under conditions described in Table 3, thereby obtaining chemicallystrengthened glasses of Example 31 to Example 38.

Examples 1 to 9 and Examples 31 to 38 are Invention Examples of thechemically strengthened glass I of the present invention and Examples 10to 15 are Comparative Examples.

Moreover, a chemical strengthening treatment was performed on glass 1 orglass 5 under conditions described in Table 4, thereby obtainingchemically strengthened glasses of Examples 16 to 26.

The following will describe the chemically strengthened glasses ofExample 1 to Example 15 and Example 31 to Example 38.

For the chemically strengthened glasses, the surface compressive stressCS (unit: MPa), thickness of the compressive stress layer DOL (unit:μm), internal tensile stress CT (unit: MPa), Compressive stress valueCS_(x) at a portion of x μm deep from glass surface (unit: MPa), area Scof the compressive stress layer (unit: MPa·μm), Sc/t (unit: MPa), areaSt of the internal tensile layer (unit: MPa·μm), St/t (unit: MPa),ΔCS₁₀₀₋₉₀ (unit: MPa/μm), CS_(DOL-20) (unit: MPa), and ΔCS_(DOL-20)(unit: MPa/μm) were measured or calculated.

CS is defined as follows by values CS_(F) and CS_(A) resulting from thefollowing two kinds of measurement methods. The same shall be applied tothe compressive stress value (CS_(x)) at a portion of x μm deep fromglass surface.

CS=CS _(F)=1.28×CS _(A)

Here, CS_(F) is a value measured by means of a surface stress meterFSM-6000 manufactured by Orihara Manufacturing Co., Ltd. and determinedby a program FsmV attached to the surface stress meter. CS_(A) is avalue measured by the aforementioned procedure using a birefringenceimaging system Abrio-IM manufactured by Tokyo Instruments, Inc. and athinned sample.

CT is defined as follows by values CT_(F) and CT_(A) resulting from thefollowing two kinds of measurement methods.

CT=CT _(F)=1.28×CT _(A)

Here, CT_(F) is a value measured by means of a surface stress meterFSM-6000 manufactured by Orihara Manufacturing Co., Ltd. and determinedby a program FsmV attached to the surface stress meter. CT_(A) is avalue measured by the aforementioned procedure using a birefringenceimaging system Abrio-IM manufactured by Tokyo Instruments, Inc. and athinned sample.

The St value is defined as follows using a value St_(F) measured bymeans of a surface stress meter FSM-6000 manufactured by OriharaManufacturing Co., Ltd. and analyzed by an attached program FsmV or avalue St_(A) obtained by the measurement using a birefringence imagingsystem Abrio-IM manufactured by Tokyo Instruments, Inc. and a thinnedsample.

St=St _(F)=1.515×St _(A)

(Indenter Indentation Test) For a chemically strengthened glass having asize of 25 mm×25 mm×0.8 mm (800 μm) in thickness, the chemicallystrengthened glass was fractured by an indenter indentation test underthe condition of holding a load ranging from 5 to 10 kgf for 15 secondswith a diamond indenter in which an indenter angle of the facing angleis 60° and the number of fragments (fragmentation number) of the glassafter fracture was measured. These results are shown in Table 2.

From these results, in the chemically strengthened glasses of Example 1to Example 9 and Example 11 to Example 14, the fragmentation number inthe fracture test by the indenter indentation test was 20 or less andthus they were highly safe glasses at the time of fracture. On the otherhand, in the chemically strengthened glasses of Example 10 and Example15, the fragmentation number greatly exceeds 20 and thus they wereglasses poor in safety at the time of fracture.

(On-Sand Drop Test)

Subsequently, for the chemically strengthened glasses, the on-sand droptest was performed by the following test method to measure an averagecrack height (unit: mm).

FIG. 3 indicates a schematic view illustrating a test method of theon-sand drop test.

First, a chemically strengthened glass 13 (50 mm×50 mm×0.8 (mm) inthickness) was attached to a hard Nylon-made mock plate 11 (50 mm×50mm×18 (mm) in thickness, weight: 54 g) through a sponge double-sidedtape 12 (#2310 manufactured by Sekisui Chemical Co., Ltd., 50 mm×50 mm×3mm in thickness) to produce a measurement sample 1. Next, 1 g of silicasand 22 (No. 5 silica sand manufactured by Takeori Company) wasscattered uniformly over a SUS plate 21 having a size of 15 cm×15 cm andthe produced measurement sample 1 in which the chemically strengthenedglass 13 is downward was dropped from a predetermined height (dropheight) to the surface of the SUS plate 21 on which silica sand 22 wasscattered. The drop test was carried out starting from a drop height: 10mm with increasing the height by 10 mm at each time, and the height atwhich the chemically strengthened glass 13 was cracked was regarded ascrack height (unit: mm). The drop test was carried out 5 to 10 times foreach Example and an average value of the crack height in the drop testwas regarded as an average crack height (unit: mm). These results areshown in Table 2.

FIG. 4 indicates a graph plotting a relationship between a compressivestress value CS₉₀ (unit: MPa) at a portion of 90 μm deep from grasssurface and an average crack height (unit: mm) for the chemicallystrengthened glass.

From FIG. 4, it is revealed that the average crack height has highcorrelation with internal compressive stress CS₉₀. When CS₉₀ is 25 MPaor more, the average crack height is about 300 mm or more and it isrevealed that a remarkable improvement in strength (acute-angle objectflaw strength) can be achieved.

Moreover, from Table 2, in the case where the value Sc/t (MPa) obtainedby dividing the area Sc (MPa·μm) of the compressive stress layer by thesheet thickness t (μm) is 28 or more, the average crack height is about300 mm or more and it is revealed that a remarkable improvement instrength (acute-angle object flaw strength) can be achieved.

<Four-Point Bending Test after Flawing or at Non-Flawing>

A chemically strengthened glass having a size of 40 mm×5 mm×0.8 (mm) inthickness was not flawed or the glass surface was flawed by pressing adiamond indenter (indenter angle of the facing angle: 110°) at a load of0.5 kgf, 1 kgf, or 2 kgf for 15 seconds. Then, a four-point bending testwas performed under conditions of an under spun of 30 mm, an upper spunof 10 mm, and a crosshead speed of 0.5 mm/minute and the fracture stress(MPa) at non-flawing and under each flawing condition was measured. Thefracture stress values (bending strength, unit: MPa) in the case ofperforming the four-point bending test at non-flawing and under eachflawing condition are shown in Table 2.

FIG. 5 indicates a graph plotting a relationship between ΔCS₁₀₀₋₉₀(unit: MPa/μm) and fracture stress (MPa) under each flawing conditionfor the chemically strengthened glasses of Examples 4, 6, 8 to 10, and15.

As mentioned above, a tensile stress of about 150 MPa is generated onthe glass surface at the time of drop of a smart phone. In the casewhere the glass has a fracture stress of 150 MPa or more, the coverglass can be prevented from cracking at the drop even after flawing.From FIG. 5, in the case where ΔCS₁₀₀₋₉₀ is 0.4 or more, it is revealedthat the bending stress after flawing at a load of 0.5 kgf or 1 kgf is150 MPa or more. Moreover, in the case where ΔCS₁₀₀₋₉₀ is 0.9 or more,it is revealed that the bending stress after flawing at 2 kgf is 150 MPaor more.

(Ion Exchange Rate)

A chemical strengthening treatment was performed on the glass 1 to glass4 under the chemical strengthening treatment conditions described in thelower column of Table 2, and DOL (μm) in each case was measured. Theseresults are collectively shown in Table 2. Incidentally, for example,“DOL (μm) at strengthening of 1 mmt KNO₃ 400° C. 1 hour” indicates DOL(μm) in the case where a chemical strengthening treatment is performedon a glass having a thickness of 1 mm under conditions of 400° C. and 1hour using a molten salt of KNO₃.

TABLE 2 No. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Glass Glass Glass Glass Glass GlassGlass Glass Glass Glass Glass Glass Glass Glass Glass Glass 1 1 1 1 1 11 1 1 1 2 2 2 3 4 Sheet thickness (μm) of glass 800 800 800 800 800 800800 800 800 800 800 800 800 843 800 for stress measurement and on-sanddrop First-stage NaNO3 (wt %) 2 2 2 3.5 100 5 7.5 10 100 5 5 5 5 50 20strengthening KNO3 (wt %) 98 98 98 96.5 95 92.5 90 95 95 95 95 50 80conditions Temperature (° C.) 400 450 450 450 425 450 450 450 450 450380 380 420 450 500 Time (h) 6 6 6 6 1.5 7.5 2.5 1.5 6 6 8 15 15 20 578Second-stage NaNO3 (wt %) 100 100 100 100 5 100 100 4 strengthening KNO3(wt %) 95 96 100 conditions Temperature (° C.) 425 425 425 425 450 425425 400 500 Time (h) 1 3 3.25 2.5 6 5 2 0.33 3 Fragmentation number 25 22 12 6 2 2 6 2 4 322 2 2 2 2 500 or mm square more CS (MPa) 616 539 562487 541 691 632 715 302 554 887 647.205 CS_(F) (MPa) 691 715 554 887647.205 DOL (pm) 115.1 127.0 123.5 130.0 173.2 164.2 129.0 119.6 123.5148.8 100.7 129.5 143.4 94.3 139.77 CT_(A) (MPa) 53.8 47.6 57.4 60.360.5 34.7 56.7 43.3 47.6 64.6 26.8 32.4 34.0 Sc_(A) (MPa · μm) 2260018807 21400 21200 19280 15770 23000 21300 20900 25810 12520 14690 12300CS_(A90) (MPa) 22.6 24.9 30.0 31.2 43.2 32.8 37.2 32.8 28.8 40.9 7.421.7 16.4 CS_(A100) (MPa) 12.9 18.4 20.3 22.8 40.4 29.5 26.7 21.1 17.832.0 3.3 14.1 13.6 Sc (MPa · μm) 34239 28493 32421 32118 29209 2389234845 32270 31664 39102 18968 22255 18635 Sc/t (MPa) 42.8 35.6 40.5 40.136.5 29.9 43.6 40.3 39.6 48.9 23.7 27.8 23.3 CS₉₀ (MPa) 28.9 31.8 38.440.0 55.3 42.0 47.6 42.0 36.9 52.3 9.4 27.8 21.0 7.4 99.7 CS₁₀₀ (MPa)16.5 23.5 26.0 29.2 51.7 37.8 34.2 27.0 22.8 41.0 4.2 18.1 17.4 78.2 CT(MPa) 68.9 61.0 73.4 77.1 77.4 44.5 72.6 55.4 60.9 82.7 34.2 41.4 43.545 113.1 CS_(DOL-20) (MPa) 25.0 17.3 20.5 16.4 10.6 14.6 23.0 27.0 21.417.2 18.6 14.1 11.1 34.3 39.2 ΔCS₁₀₀₋₉₀ (MPa/Mm) 1.25 0.83 1.24 1.080.36 0.42 1.34 1.50 1.41 1.14 0.52 0.97 0.36 2.15 ΔCS_(DOL-20) (MPa/Mm)1.25 0.86 1.02 0.82 0.53 0.73 1.15 1.35 1.07 0.86 0.93 0.70 0.56 1.721.96 0.8 t sand average crack 413 460 478 497 360 456 313 406 488 509256 222 150 600 height 0.8 t 4PB average (MPa) 597 535 554 493 536 647610 684 345 550 653 0.8 t flawing Flawing at 311 194 393 339 383 616 4PBaverage 0.5 kgf (MPa) Flawing at 1 239 175 350 305 307 344 kgf Flawingat 2 kgf 128 113 208 192 210 330 DOL (μm) at strengthening 76.9 58 of 1mmt NaNO₃ 400° C. 1 hour DOL (μm) at strengthening 21 5 or of 1 mmt KNO₃400° C. 1 hour less DOL (μm) at strengthening 92.8 of 1 mmt NaNO₃ 425°C. 1 hour DOL (μm) at strengthening 5 or of 1 mmt KNO₃ 425° C. 1 hourless

TABLE 3 No. Ex. 31 Ex. 32 Ex. 33 Ex. 34 Ex. 35 Ex. 36 Ex. 37 Ex. 38Glass Glass Glass Glass Glass Glass Glass Glass Glass 6 6 6 6 7 7 7 7Sheet thickness (μm) 800 800 800 800 800 800 800 800 of glass for stressmeasurement and on-sand drop First-stage NaNO3 (wt %) 100 100 100 100100 100 100 100 strengthening KNO3 (wt %) conditions Temperature 450 450450 450 450 450 450 450 (° C.) Time (h) 1 1 1 1 1 1 1 1 Second-stageNaNO3 (wt %) 1 2 5 1 2 5 strengthening KNO3 (wt %) 100 99 98 95 100 9998 95 conditions Temperature 450 450 450 450 450 450 450 450 (° C.) Time(h) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Fragmentation number 2 2 2 2 2 2 2 225 mm square CS (MPa) 909.955 861.78 859.4 792.156 884.678 850.817821.24 749.23 CS_(F) (MPa) 909.955 861.78 859.4 792.156 884.678 850.817821.24 749.23 DOL (μm) 149 143 147 142 134 139 135 132 CT_(A) (MPa) 43.948.9 44.6 52.6 37.6 38.0 38.3 41.3 Sc_(A) (MPa · μm) 15195 19443 1619121666 12935 13516 13637 16221 CS_(A90) (MPa) 36.6 42.3 38.9 47.1 24.327.7 29.4 30.2 CS_(A100) (MPa) 30.8 34.3 32.2 37.5 18.4 21.3 21.6 22.8Sc (MPa · μm) 23020 29456 24529 32824 19597 20477 20660 24574 Sc/t (MPa)28.8 36.8 30.7 41.0 24.5 25.6 25.8 30.7 CS₉₀ (MPa) 46.8 54.2 49.8 60.231.1 35.5 37.6 38.6 CS₁₀₀ (MPa) 39.5 43.9 41.3 48.1 23.5 27.3 27.6 29.2CT (MPa) 56.2 62.6 57.1 67.3 48.1 48.6 49.0 52.9 CS_(DOL-20) (MPa) 13.819.6 15.3 19.0 13.6 13.5 15.1 17.6 ΔCS₁₀₀₋₉₀ (MPa/μm) 0.74 1.03 0.851.22 0.76 0.82 1.00 0.94 ΔCS_(DOL-20) (MPa/μm) 0.69 0.98 0.77 0.95 0.680.68 0.76 0.88 0.8 t sand average crack height 0.8 t 4PB average (MPa)0.8 t flawing Flawing at 0.5 4PB average kgf (MPa) Flawing at 1 kgfFlawing at 2 kgf DOL (μm) at strengthening of 1 mmt NaNO₃ 400° C. 1 hourDOL (μm) at strengthening of 1 mmt KNO₃ 400° C. 1 hour DOL (μm) atstrengthening of 1 mmt NaNO₃ 425° C. 1 hour DOL (μm) at strengthening of1 mmt KNO₃ 425° C. 1 hour

The following will describe the chemically strengthened glasses ofExample 16 to Example 25.

(Measurement of CS_(x))

For each of the chemically strengthened glasses of Examples 16 to 25,the compressive stress value CS_(x) (unit: MPa) at a portion of x μmdeep from the glass surface was measured by a method using theaforementioned birefringence imaging system Abrio-IM manufactured byTokyo Instruments, Inc. and a thinned sample. Results thereof are shownin Table 4. For the chemically strengthened glass of Example 26, CS andDOL were measured by a surface stress meter (FSM-6000 manufactured byOrihara Manufacturing Co., Ltd.). CS was 95.7 MPa and DOL was 34.6 μm.

(Measurement by EPMA)

For the chemically strengthened glasses, each of the following N0, NM,Nh, K0, KM, and Kh were calculated from the Na ion concentration profileand the K ion concentration profile obtained by using an electron probemicroanalyzer (Electron Probe Micro Analyzer: EPMA), and were shown inTable 1. A cross-section of each chemically strengthened glass wasmirror-polished and was coated with C in about 30 nm, and then theresultant glass was subjected to the EPMA analysis. As EPMA, JXA-8500Fmanufactured by JOEL Ltd. was used and a profile of the number of countsof characteristic X-rays for each element was obtained at anacceleration voltage of 15 kV, a probe current of 30 nA, a probediameter of 1 μmϕ, a step interval of 1 μm, a measurement time of 1 s, adispersive crystal TAPH (Na Kα ray, peak position: 129.55 mm), and PETH(K Kα ray, peak position: 119.75 mm). Further, the number of counts ofeach element at a center of the sheet thickness of the sample wasconverted to % by weight in the sample composition to prepare an ionconcentration profile.

N0: a value obtained by converting a value on the outermost surface inan Na ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight)

NM: a value obtained by converting a maximum value in an Na ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to Na₂O (% by weight)

Nh: a value obtained by converting a central value of sheet thickness inan Na ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to Na₂O (% by weight)

K0: a value obtained by converting a value on the outermost surface in aK ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to K₂O (% by weight)

KM: a value obtained by converting a maximum value in a K ionconcentration profile in a sheet thickness direction of the chemicallystrengthened glass measured by EPMA to K₂O (% by weight)

Kh: a value obtained by converting a central value of sheet thickness ina K ion concentration profile in a sheet thickness direction of thechemically strengthened glass measured by EPMA to K₂O (% by weight).

Moreover, from the above results, N0/Nh, NM/Nh, N0/NM, N0−Nh, NM−Nh,N0−NM, K0/Kh, KM/Kh, K0/KM, and KM−Kh were each calculated and shown inTable 1.

<Four-Point Bending Test after Flawing or at Non-Flawing>

A diamond indenter (indenter angle of the facing angle: 110°) waspressed to a chemically strengthened glass at a load of 0.5 kgf, 1 kgf,1.5 kgf, or 2 kgf for 15 seconds, thereby flawing the glass surface.Then, a four-point bending test was performed under conditions of anunder spun of 30 mm, an upper spun of 10 mm, and a crosshead speed of0.5 mm/minute and fracture stress (MPa) under each flawing condition wasmeasured. The fracture stress values (bending strength, unit: MPa) inthe case of performing the four-point bending test at non-flawing and ateach load of the indenter indentation were shown in Table 1.

(On-Sand Drop Test)

For the chemically strengthened glasses, the on-sand drop test wasperformed by the aforementioned test method to measure an average crackheight (unit: mm). Results are shown in Table 4.

As for Examples 16, 18, 21, 22, 24, and 25, DOL was 100 μm or more inall Examples and the average crack height in the above on-sand drop testwas 300 mm or more in all cases. In Example 26, DOL was 100 μm or lessand the average crack height was 129 mm. It is shown that, in the casewhere DOL is 100 μm or more, the strength can be improved in theresistance in the on-sand drop test and thus it is revealed that animprovement in strength, which is necessary for a cover glass, againstfracture resulting from an acute-angle object flawing can be attained.

(Intender Indentation Test)

For a chemically strengthened glass having a size of 25 mm×25 mm×0.8 mmin sheet thickness, by an indenter indentation test under the conditionof holding a load ranging from 3 to 10 kgf for 15 seconds with a diamondindenter in which an indenter angle of the facing angle is 60°, thechemically strengthened glass was fractured and the fragmentation numberof the glass after fracture was measured. Results thereof are shown inTable 4.

TABLE 4 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22 Ex. 23 Ex. 24Ex. 25 Ex. 26 Glass composition Glass Glass Glass Glass Glass GlassGlass Glass Glass Glass Glass 1 1 1 1 1 1 1 1 1 1 5 Sheet thickness mm0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.825 First- KNO3 wt % 95 100100 100 90 96.5 95 98 95 100 stage NaNO3 wt % 5 100 10 3.5 5 2 5 1 Temp.450 450 425 450 450 450 450 450 450 450 450 time 7.5 6 1.5 6 6 1.5 6 6 66 4 Second- KNO3 wt % — 95 — stage NaNO3 wt % — 100 5 100 100 — 100 100100 100 Temp. — 425 450 425 425 — 425 425 425 425 time — 1.5 6 4 6 — 2.51.5 3 2 Abrio CS@0 495 575 461 323 323 427 451 625 552 672 CS@10 96 147141 157 157 179 215 189 148 234 CS@20 53 48 74 135 135 153 122 117 104140 CS@30 57 37 75 117 117 131 99 100 90 118 CS@40 55 30 73 103 103 10984 86 74 101 CS@50 50 16 67 85 85 91 71 72 64 87 CS@60 45 8 63 70 70 7559 63 54 75 CS@70 40 3 57 53 53 60 47 50 41 61 CS@80 35 −4 51 46 46 4539 43 32 51 CS@90 31 −12 43 36 36 33 30 37 25 44 CS@100 27 −9 40 23 2321 21 30 18 34 CS@110 25 −11 35 17 17 11 14 23 13 26 CS@120 19 −17 28 77 1 7 18 6 20 CS@130 14 −16 22 4 4 −8 1 12 −3 12 CS@140 11 −16 18 −13−13 −14 −4 7 −7 7 CS@150 8 −16 13 −14 −14 −20 −9 2 −11 −2 CS@160 4 −19 7−20 −20 −26 −14 −5 −14 −6 CS@170 −3 −20 4 −23 −23 −30 −18 −10 −14 −12CS@180 −4 −21 −1 −29 −29 −32 −22 −13 −19 −16 CS@190 −6 −19 −8 −35 −35−36 −26 −18 −21 −20 0.8 t 4PB average 647 684 493 550 (MPa) 0.8 t flaw0.5 kgf 194 393 311 383 4PB average   1 kgf 175 350 239 307 (MPa)   2kgf 113 208 128 210 ΕPMA N0 Na₂O 1.72 3.40 2.69 4.57 6.63 5.05 4.71 5.135.36 wt % @0 μm NM Na₂O 5.28 4.25 5.86 6.16 7.26 7.54 6.15 6.98 6.47 wt%_MAX Nh Na₂O 3.07 3.06 2.96 2.97 2.92 3.08 2.96 2.93 3.08 wt % @(t/2)pm N0/Nh 0.56 1.11 0.91 1.54 2.27 1.65 1.59 1.75 1.74 NM/Nh 1.72 1.391.98 2.07 2.49 2.46 2.08 2.39 2.10 N0/NM 0.32 0.80 0.46 0.74 0.91 0.670.77 0.73 0.83 K0 K₂O 8.11 7.59 8.74 5.78 5.22 5.10 5.69 5.37 5.58 wt %@0 μm KM K₂O 8.15 7.65 8.74 5.86 5.24 5.10 5.69 5.37 5.58 wt %_MAX KhK₂O 1.55 1.50 1.50 1.51 1.59 1.51 1.51 1.56 1.52 wt % @(t/2) μm K0/Kh5.24 5.06 5.84 3.83 3.30 3.38 3.78 3.44 3.67 KM/Kh 5.27 5.10 5.84 3.873.31 3.38 3.78 3.44 3.67 K0/KM 1.00 0.99 1.00 0.99 1.00 1.00 1.00 1.001.00 Fragmentation 2 2 2 2 125 2 2 203 2 322 2 number 25 mm squareOn-sand drop 456 360 406 497 460 509 129 Average crack height mm

FIG. 6 shows a graph plotting a relationship between bending strengthafter flawing under a condition of a load of 0.5 kgf or 1 kgf and CS₂₀.FIG. 7 shows a plot of a relationship between bending strength afterflawing under a condition of a load of 0.5 kgf or 1 kgf and CS₄₀. Asmentioned before, in the case where where the bending strength afterflawing is 200 MPa or more, the failure rate in the case of the use as acover glass of a smart phone or a tablet PC can be improved. Also asunderstood from FIG. 6 and FIG. 7, there is a strong correlation betweenthe compressive stress values (CS₂₀ to CS₄₀) at a portion of 20 to 40 μmdeep from the glass surface and the bending strength after flawing and,as the compressive stress values at a portion of 20 to 40 μm deep fromthe glass surface increases, a higher bending strength after flawingtends to be obtained. It is revealed that, in the case where CS₂₀ orCS₄₀ is 60 MPa or more, a bending strength after flawing of 200 MPa ormore can be achieved.

FIG. 8 shows a graph plotting a relationship between NM/Nh and CS₂₀ orCS₄₀. According to this, as NM/Nh increases, CS₂₀ and CS₄₀ tend toincrease. It is revealed that, in the case where NM/Nh is 1.8 or more,CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, a bendingstrength after flawing of 200 MPa or more can be achieved.

FIG. 9 shows a graph plotting a relationship between N0/Nh and CS₂₀ orCS₄₀. According to this, as N0/Nh increases, CS₂₀ and CS₄₀ tend toincrease. It is revealed that, in the case where N0/Nh is 0.8 or more,CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, a bendingstrength after flawing of 200 MPa or more can be achieved.

FIG. 10 shows a graph plotting a relationship between N0/NM and CS₂₀ orCS₄₀. According to this, as N0/NM increases, CS₂₀ and CS₄₀ tend toincrease. It is revealed that, in the case where N0/NM is 0.4 or more,CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, a bendingstrength after flawing of 200 MPa or more can be achieved.

The chemically strengthened glasses of Examples 16, 21, and 22 had aKM/Kh of 3 or more and had a bending strength of 400 MPa or more that isrequired from the viewpoint of securing the cover glass strength.

FIG. 11 shows a graph plotting a relationship between NM−Nh (unit: wt %)and CS₂₀ or CS₄₀. According to this, as NM−Nh increases, CS₂₀ and CS₄₀tend to increase. It is revealed that, in the case where NM−Nh is 2.2 wt% or more, CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, abending strength after flawing of 200 MPa or more can be achieved.

FIG. 12 shows a graph plotting a relationship between N0−Nh and CS₂₀ orCS₄₀. According to this, as N0−Nh increases, CS₂₀ and CS₄₀ tend toincrease. It is revealed that, in the case where N0−Nh is −0.4 wt % ormore, CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, abending strength after flawing of 200 MPa or more can be achieved.

FIG. 13 shows a graph plotting a relationship between N0−NM and CS₂₀ orCS₄₀. According to this, as N0−NM increases, CS₂₀ and CS₄₀ tend toincrease. It is revealed that, in the case where N0−NM is −3.5 wt % ormore, CS₂₀ and CS₄₀ can be controlled to 60 MPa or more, that is, abending strength after flawing of 200 MPa or more can be achieved.

The chemically strengthened glasses of Examples 16, 21, and 22 had aKM−Kh of 3 wt % or more and had a bending strength of 400 MPa or morethat is required from the viewpoint of securing the cover glassstrength.

While the invention has been described in detail and with reference tospecific embodiments thereof, it is apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope of the present invention.

The present application is based on Japanese Patent Application (No.2016-010002) filed on Jan. 21, 2016, Japanese Patent Application (No.2016-204746) filed on Oct. 18, 2016, and Japanese Patent Application(No. 2016-204747) filed on Oct. 18, 2016 and the whole contents areincorporated herein by reference.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   1. Measurement sample    -   11. Mock plate    -   12. Sponge double-sided tape    -   13. Chemically strengthened glass    -   21. SUS plate    -   22. Silica sand

1. A chemically strengthened glass having a thickness t of 2 mm or less,having a compressive stress value (CS₉₀) at a portion of 90 μm deep froma glass surface being 25 MPa or more, and satisfying a number offragments generated within a size of 25 mm×25 mm being 20 or less in afracture test according to an indenter indentation test under acondition of holding a load ranging from 5 kgf to 10 kgf for 15 secondswith a pyramidal diamond indenter having an indenter angle of a facingangle of 60°, wherein the chemically strengthened glass provides a depthof a compressive stress layer (DOL) being 50 μm or more when a glasssheet having a thickness of 1 mm which has a matrix composition of thechemically strengthened glass and has been gradually cooled from atemperature T° C., which is 30° C. to 50° C. higher than a glasstransition temperature, to (T−300)° C. at 0.5° C./minute, is subjectedto an ion exchange treatment for one hour with a molten salt at 400° C.composed of KNO₃, NaNO₃, or a mixed salt of KNO₃ and NaNO₃.
 2. Thechemically strengthened glass according to claim 1, satisfying ΔCS₁₀₀₋₉₀(unit: MPa/μm) being 0.4 or more, wherein ΔCS₁₀₀₋₉₀ is calculatedaccording to the following expression using CS₉₀ and a compressivestress value (CS₁₀₀) at a portion of 100 μm deep from the glass surface,ΔCS ₁₀₀₋₉₀=(CS ₉₀ −CS ₁₀₀)/(100−90).
 3. The chemically strengthenedglass according to claim 2, wherein ΔCS₁₀₀₋₉₀ (unit: MPa/μm) is 4.0 orless.
 4. The chemically strengthened glass according to claim 1,satisfying ΔCS_(DOL-20) (unit: MPa/μm) being 0.4 or more, whereinΔCS_(DOL-20) is calculated according to the following expression using acompressive stress value CS_(DOL-20) at a depth closer to a glasssurface side by 20 μm from DOL,ΔCS _(DOL-20) =CS _(DOL-20)/20.
 5. The chemically strengthened glassaccording to claim 4, wherein ΔCS_(DOL-20) (unit: MPa/μm) is 4.0 orless.
 6. The chemically strengthened glass according to claim 1, havinga fracture stress value σa (bending strength, unit: MPa) being 150 MPaor more obtained by performing a four-point bending test underconditions of an under spun of 30 mm, an upper spun of 10 mm, and acrosshead speed of 0.5 mm/minute after the glass surface is flawed bypressing a diamond indenter (indenter angle of facing angle: 110°)thereto for 15 seconds at a load of 0.5 kgf.
 7. The chemicallystrengthened glass according to claim 1, having a fracture stress valueσc (bending strength, unit: MPa) being 150 MPa or more obtained byperforming a four-point bending test under conditions of an under spunof 30 mm, an upper spun of 10 mm, and a crosshead speed of 0.5 mm/minuteafter the glass surface is flawed by pressing a diamond indenter(indenter angle of facing angle: 110°) thereto for 15 seconds at a loadof 2 kgf.
 8. The chemically strengthened glass according to claim 1,having a surface compressive stress value being 300 MPa or more.
 9. Thechemically strengthened glass according to claim 1, wherein the depth ofa compressive stress layer (DOL) is 100 μm or more.
 10. The chemicallystrengthened glass according to claim 1, wherein the thickness t is 0.9mm or less.
 11. The chemically strengthened glass according to claim 1,having an area Sc (MPa·μm) of a compressive stress layer being 20,000MPa·μm or more.
 12. The chemically strengthened glass according to claim1, satisfying a value Sc/t (MPa) obtained by dividing an area Sc(MPa·μm) of a compressive stress layer by the thickness t (μm) being 28MPa or more.
 13. The chemically strengthened glass according to claim 1,having a matrix composition comprising, in terms of molar percentage onthe basis of oxides, from 50 to 80% of SiO₂, from 1 to 30% of Al₂O₃,from 0 to 5% of B₂O₃, from 0 to 4% of P₂O₅, from 3 to 20% of Li₂O, from0 to 8% of Na₂O, from 0 to 10% of K₂O, from 3 to 20% of MgO, from 0 to20% of CaO, from 0 to 20% of SrO, from 0 to 15% of BaO, from 0 to 10% ofZnO, from 0 to 1% of TiO₂, and from 0 to 8% of ZrO₂.
 14. A method formanufacturing the chemically strengthened glass according to claim 1,comprising performing at least two stages of ion exchange treatments onthe glass, wherein KNO₃ concentration in a molten salt to be used in afirst-stage ion exchange treatment is 60% by weight or more and NaNO₃concentration in a molten salt to be used in a second-stage ion exchangetreatment is 5% by weight or more.